Subject: Anomalous Phonons and High Temperature Superconductivity in Copper Oxides

Note: change of time and location for seminar, this week only.

It is well known that electron-phonon coupling is responsible for superconductivity in conventional superconductors, but the prevailing view is that it is not important in high temperature superconductivity. Yet, evidence that electron-phonon coupling is very strong for certain phonons in the copper oxides has been building. In particular, Cu-O bond-stretching phonons at 65-85meV in La2−xSrxCuO4 are known to show anomalously large broadening and softening near the reduced wavevector q=(0.3,0,0). Recently we systematically investigated spectral functions of these phonons by inelastic neutron and x-ray scattering and measured dispersions of electrons to which these phonons should be coupled by angle resolved photoemission (ARPES). These electronic dispersions have kinks around 70 meV that are typically attributed to coupling of electrons to a bosonic mode (which could be a phonon) that mediates superconductivity. Remarkably, we found that the kinks remain strong in the heavily overdoped region of the doping phase diagram of La2−xSrxCuO4, even when the superconductivity completely disappears. We also found that doping dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e., the two phenomena are not connected. In fact, while the Cu-O bond stretching phonons show giant electron-phonon effects, there are no features in the electronic dispersions of the same samples that can be attributed to these phonons. We show that these results provide indirect evidence that the phonon anomaly originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity. I will also discuss earlier results on a copper oxide with a very high Tc, YBa2Cu3O7, where a similar phonon anomaly becomes greatly enhanced in the superconducting state.

Speaker: Christian Wuthrich, Department of Philosophy, University of California, San Diego

Subject: Space and Time from Causality

Refreshments served in Room 275 Nicholson Hall at 3:15 p.m.

Space and time are conspicuous by their absence in fundamental theories of quantum gravity. Causal set theory is such a theory. It follows an eminent tradition of reducing spatiotemporal relations to causal ones. I will illustrate how the causal sets lack all spatial and most temporal structure. The absence of spacetime from the fundamental level of reality poses, however, a deep philosophical and scientific challenge. On the philosophical side, the threat of empirical incoherence looms. The scientific aspect arises from the need for any novel theory to explain the success, such as it was, of the theory it seeks to depose. Both sides of the challenge are resolved if we articulate a physically salient recovery of relativistic spacetime from the underlying fundamental causal sets. I will sketch ways in which this can be achieved.

Cells have the remarkable ability to accurately sense chemical gradients over a wide range of concentrations. The process of chemotaxis and chemotropism has been extensively studied in the past and it has been shown that adaptation is a crucial feature of the underlying signaling networks. Although many signaling proteins responsible for gradient sensing have been identified, it is unclear whether or how their spatial distribution can modulate pathway activity. Here, we show that spatial clustering of a signaling complex presents an additional regulatory layer that actively tunes pathway gain. We demonstrate that clustering of the signaling complex itself activates the pathway bypassing receptor activation and that the degree of clustering correlates with the adaptive output. Furthermore we identify a negative feedback, which acts on the degree of clustering and which allows adaptation to the input stimulus. Our results may present a general principle of how cells use spatial re-distribution of its signaling proteins as an additional regulatory layer to tune pathway activity.

Recent interest in composite materials based on hydrogenated amorphous silicon (a-Si:H) stems in part from its potential for technical applications in thin film transistors and solar cells. Previous reports have shown promising results for films of a-Si:H with embedded silicon nanocrystals, with the goal of combining the low cost, large area benefits of hydrogenated amorphous silicon with the superior electronic characteristics of crystalline material. These materials are fabricated in a dual-chamber plasma-enhanced chemical vapor deposition system in which the nanocrystals are produced separately from the amorphous film, providing the flexibility to independently tune the growth parameters of each phase; however, electronic transport through these and other similar materials is not well understood. This thesis reports the synthesis and characterization of thin films composed of germanium nanocrystals embedded in a-Si:H. The results presented here describe detailed measurements of the conductivity, photoconductivity and thermopower which reveal a transition from conduction through the a-Si:H for samples with few germanium nanocrystals, to conduction through the nanocrystal phase as the germanium crystal fraction XGe is increased. These films display reduced photosensitivity as XGe is increased, but an unexpected increase in the dark conductivity is found in samples with XGe > 5% after long light exposures.

Gases of atoms can be cooled to temperatures close to absolute zero, where intriguing quantum behaviors such as Bose-Einstein condensation and superfluidity emerge. A new direction in experiments is to try to produce an ultracold gas of molecules, rather than atoms. In particular, polar molecules, which have strong dipole-dipole interactions, are interesting for applications ranging from quantum information to modeling condensed matter physics. I will describe experiments that produce and explore an ultracold gas of polar molecules.

In the absence of New Physics at the TeV scale, GUTs still provide a
good motivation for supersymmetry at higher scales. Notably, it is
typically non-trivial to UV complete GUTs into string theory, but one
promising possibility is found in F-theory. I shall argue, therefore,
that considerations from string theory should play a central role in
the construction of such models of high scale supersymmetry.
Specifically, F-theory GUTs lead to calculable UV threshold
corrections to the running of the gauge couplings, which can in
principle improve the precision of unification. I will examine the
prospect of precision unification in models of F-theory High Scale
Supersymmetry and the experimental constraints on this model
from the non-observation of proton decay. Further, I will discuss
to what extent the proton lifetime can be extending due to the
localization of X,Y gauge bosons in higher dimensions.

Ultracold quantum gases of atoms are model systems that provide access to strongly correlated many-body physics as well as to intriguing few-body physics phenomena. It is particularly interesting to explore the behavior of quantum gases with infinitely strong interactions (the so-called unitary gas). This regime has been experimentally studied for Fermi gases of atoms but is difficult to access for an ultracold gas of bosons. I will discuss a recent experiment where we quickly took a Bose-Einstein condensate to this regime of strong interactions and looked at the ensuing dynamics in this non-equilibrium system.

Gravity models of spatial interaction, which provide quantitative estimates of the decline in intensity of economic and social interactions with distance, are now ubiquitous in urban and transportation planning, international trade, and many other areas. They were discovered through analysis of a unique large data set by a Belgian engineer in 1846, at the height of the British Railway Mania. They contradicted deeply embedded beliefs about the nature of demand for railway service, and had they been properly applied, they could have lessened the investment losses of that bubble. A study of the information flows in Britain, primarily in the newspaper press, provides an instructive picture of slow diffusion of significant factual information, the distortions it suffered, and the wrong conclusions that were drawn from the experience in the end.

From the whopping magnetic fields surrounding neutron stars to the feeble crustal fields of Mars, magnetized bodies and their magnetospheres are present throughout the universe. Although the outer boundary, or the magnetopause, often shields the magnetized body from the surrounding space environment, energy can sometimes pass through and cause significant disturbances. At the Earth, the understanding developed over the past several decades is that the energy coupled in from the flowing solar wind is purely a function of the conditions within the solar wind. In contrast to this, I will present new spacecraft observations from the THEMIS mission as well as ground-based measurements suggesting the magnetosphere, with the help of a plasmaspheric plume, is defending itself from intense solar activity. In this framework, the internal conditions within a magnetosphere can impact and even control how energy is coupled in from the outside environment.

The atmosphere of the Sun (and that of many another star) hosts catastrophic disruptions - flares - that involve a very wide range of phenomena. Much of the development of our basic understanding began with John Winckler (Minnesota) and his students, and their students. The physics of space plasmas plays a major role in modern analysis. I will discuss new wrinkles in the physics of flares, emphasizing the X-ray observations, and assess some recent new information regarding extreme events - the "black swans" - detected now in tree-ring fossil records and Kepler astronomical photometry.

We introduce and systematically study an expansive class of "orbifold Higgs" theories in which the weak scale is protected by accidental symmetries arising from the orbifold reduction of continuous symmetries. The protection mechanism eliminates quadratic sensitivity of the Higgs mass to higher scales at one loop (or more) and does not involve any new states charged under the Standard Model.

The development of collective long-range order occurs by the spontaneous breaking of fundamental symmetries, but the broken symmetry that develops below 17.5K in the heavy fermion material URu2Si2 has eluded identification for over twenty five years – while there is clear mean-field-like specific heat anomaly, the absence of any large observable order parameter has given the problem the name "hidden order." In this talk, I will show how the recent observation of heavy Ising quasiparticles in the hidden order phase provides the missing puzzle piece. To form Ising quasiparticles, the conduction electrons must hybridize with a local Ising moment - a 5f2 state of the uranium atom with integer spin. As the hybridization mixes states of integer and half-integer spin, it is itself a spinor and this ``hastatic'' (hasta: [Latin] spear) order parameter therefore breaks both time-reversal and double time-reversal symmetries. A microscopic theory of hastatic order naturally unites a number of disparate experimental results from the large entropy of condensation to the spin rotational symmetry breaking seen in torque magnetometry. Hastatic order also has a number of experimental consequences, most notably a tiny transverse magnetic moment in the conduction electrons.

The space environment provides a near-pristine laboratory for studying plasma physics. /In situ/ particle instrumentation complemented by electromagnetic fields can be used to study plasma dynamics at all scales, ranging from astronomical units to individual particle motion. In addition to scientific expertise, detailed technical knowledge of instrument design and behavior is paramount for finding signal in the noise and maximizing the scientific return of space missions. Here, recent data from the MESSENGER spacecraft is used to demonstrate how penetrating radiation that inhibits nominal plasma measurements can be used to infer magnetic topology and remotely probe the smallest scales in Mercury’s space environment.Such structure will be measured in great detail at Earth with the Fast Plasma Investigation suite on the upcoming Magnetospheric Multiscale Mission (MMS). Key engineering innovations in both laboratory and in-flight calibration activities will be presented that will enable MMS to provide the highest ever spatial and temporal resolution measurements of space plasmas.

Physicists have been interested in the cyclic competition of species for quite some time. For the first part of my presentation I broadly introduce population dynamics, game theory, and pattern formation observed in spatial systems. I then present the results for when four species compete on different lattice structures. It is found that the probability distributions of the extinction times are non-trivial and contain more information than mean extinction times which are commonly reported in the literature. These non-trivial features have their origin in domain formation of both individual species and alliances and promote coexistence in the system. In the last part of my talk the agents are allowed to have mixed strategies and choose their strategy out of a distribution. This scheme for choosing strategies is not often seen in biology, however it does have applicabilities to economic systems such as the public goods game. This is simulated on a 1D lattice with three and four strategies and interesting patterns and stability properties are found depending on how discretized the choice of strategy of the agents is.

The observation that the three types of neutrino flavor oscillate among themselves led to the realisation that neutrinos have a very small but non-zero mass. This is extremely important because the supremely successful Standard Model of particle physics had expected, and indeed needed, the neutrinos to have exactly zero mass. Since the discovery of neutrino oscillations over the last 15 years, the parameters of the oscillations have been sufficiently well measured to turn neutrino oscillations into a tool for learning more about the elusive neutrino. I will explain the concept of neutrino oscillations, and report on the recent results from around the world in context with the new challenges now facing researchers of inferring the remaining unknown neutrino properties. I will talk briefly about an exciting new project on the horizon for the very near future.

The IceCube collaboration has recently observed high energy neutrinos in the 30 TeV to 2 PeV range. The flux is much above the atmospheric neutrino background and requires new sources to explain its origin. In this talk, I will provide a list of potential explanations with a focus on decaying dark matter and point-like sources.

In recent years, anisotropic electronic phases have been discovered in a variety of strongly correlated quantum materials. Borrowing language from the field of soft condensed matter physics, such phases are referred to as electronic nematic phases when they are driven by electron correlations and break a discrete rotational symmetry of the crystal lattice without further breaking translational symmetry. In this talk I'll outline a new technique that we have developed based on elastoresistance measurements, which probes an associated quantity, the nematic susceptibility. Measurements of this quantity directly reveal the presence of an electronic nematic phase transition in underdoped Fe-based superconductors, and an associated quantum phase transition near optimal doping (i.e. the doping that yields the maximum critical temperature of the superconductor). I'll explain the possible significance of this observation. I'll also discuss the case of ferroquadrupolar order in 4f intermetallic systems.

Two-dimensional AKLT model on a honeycomb lattice has been shown to be a universal resource for quantum computation. In this valence bond solid, however, the spin interactions involve higher powers of the Heisenberg coupling (S_i *S_j)^n, making these states seemingly unrealistic on bipartite lattices,
where one expects a simple antiferromagnetic order. We show that those interactions can be generated by orbital physics in multiorbital Mott insulators. We focus on t_{2g} electrons on the honeycomb lattice and propose a physical realization of the spin-3/2 AKLT state. We find a phase transition from the AKLT to the Neel state on increasing Hund's rule coupling, which is confirmed by density matrix renormalization group (DMRG) simulations. An experimental signature of the AKLT state consists of protected, free spins-1/2 on lattice vacancies, which may be detected in the spin susceptibility.

Massive star formation is still poorly understood. These stars evolve quickly and have a profound effect on their immediate environment, yet they often form dense clusters of OB stars. Using SOFIA Mid-IR imaging, Hershel photometry, CSO Sub-mm polarimetry, and the GPIPS database, we explore the nature of massive star formation in the G034.43+00.24 Infrared Dark Cloud. In particular, we investigate a luminous Class 0 YSO (MM1), which is embedded in a very dense molecular cloud core thousands of visual magnitudes thick.

Speaker: Craig Hassel, Department of Food, Science and Nutrition, University of Minnesota

Subject: Spanning Cultural Difference in Food and Health

Refreshments served in Room 275 Nicholson Hall at 3:15 p.m.

I will explore examples of University outreach/cross-cultural engagement with older, non-biomedical thought systems (African, Chinese Medicine, Indigenous knowledge traditions) bringing profound cultural difference in epistemology and ontology. Spanning these chasms of cultural difference involves cognitive bridge-building, a form of community engaged scholarship wherein habitual attachment to familiar, self-affirming, biomedical mental models is relaxed, allowing for temporary dwelling within unfamiliar, and often unsettling assumptive terrain. Perseverance with such bridge-building creates novel cognitive locations and perceptual lenses through which to reconsider disciplinary issues of the day and to illuminate otherwise opaque cultural/disciplinary "hidden subjectivities" that too often escape conscious attention and peer review. I refer back to nutrition science with its positivist legacy, its history of success with deterministic, acute deficiency disease, and its current struggle with more complex diet-related chronic disease and concepts of well being. I propose that nutrition as a biomedical science would advance by learning and adapting discourses and/or thought styles akin to those within the humanities and/or social sciences.

Subject: A Systems Approach to Teaching: Why the Introductory Physics Courses at Minnesota Work and How They Can Fail.

This seminar will be a discussion of the parts that go into the structure of the
introductory physics courses at the University of Minnesota. The design of these courses is based on empirical studies and general principles of cognitive science to fit the constraints and institutional goals of the University. We will discuss the goals of the course, the pedagogy employed and the impact of the structure on students, TAs, and faculty. Within this learning framework important considerations are the role of the faculty, TAs, and students and the purpose of lectures, laboratories, discussion sections, problems, and tests. We will also discuss how the outcomes of the course can be changed, for better or worse, and what changes cause it to fail.

Subject: Exploring the Sun at high energies: progress and new instrumentation

The Sun provides a nearby case study in which to study plasma processes and high-energy astrophysical phenomena with high-resolution remote sensing combined with in-situ data and even multiviewpoint measurements — tools that are not available to study any object outside the solar system. In addition to basic physics research interests, understanding high-energy aspects of the Sun also has practical applications, since Earth-directed solar eruptive events can pose a danger to satellites, astronauts, and power grids. New, direct-focusing techniques are now available to study solar flares and eruptions using hard X-rays. The first generation of solar-dedicated hard X-ray focusing optics has recently flown on suborbital missions (rockets and balloons). And from low-Earth orbit, the Nuclear Spectroscopic Array (NuSTAR), a direct-focusing instrument designed to look at the faintest objects outside the solar system, has also produced detailed hard X-ray images of the Sun. This seminar will cover recent advances in high-energy solar flare physics and will present new instrumentation, with emphasis on NuSTAR solar observations and on the FOXSI solar sounding rocket.

Subject: The centenary of general relativity: How did Einstein find his gravitational field equations?

Refreshments served in Room 216 Physics after colloquium

In his search for gravitational field equations from late 1912 to late 1915, Einstein vacillated between two different strategies. Following a "mathematical strategy," he extracted candidate field equations from the Riemann curvature tensor and checked whether these equations were compatible with energy-momentum conservation and reproduced Newton’s theory of gravity in the appropriate limit. Following a "physical strategy," he constructed field equations for the gravitational field in close analogy with those for the electromagnetic field. In his later years, Einstein routinely claimed that he brought his search for gravitational field equations to a successful conclusion in November 1915 by switching to the mathematical strategy at the eleventh hour. Most commentators have accepted this later assessment but we have argued that Einstein achieved his breakthrough of November 1915 by doggedly pursuing the physical strategy. In a lecture in Vienna in September 1913, Einstein clearly laid out this physical strategy. As long as one took the older Einstein’s word for it that the mathematical strategy was responsible for the success of November 1915, one could quickly pass over the Vienna lecture. But if it was really the physical strategy that was responsible for this success, as we believe, the Vienna lecture deserves a much more prominent place in the account of the genesis of general relativity than it has been given so far.

In recent years, there has been much progress in understanding the ultraviolet properties of supergravity amplitudes. Through symmetry arguments and direct computations, supergravity has been found to be much tamer in the ultraviolet than previously thought. In this talk, I will discuss a new type of cancellation known as enhanced cancellation that renders a supergravity amplitude finite. Such cancellations cannot be understood through any symmetry argument based on a local covariant diagrammatic formalism. I will provide several examples where we observe enhanced cancellations, showing finiteness in supergravity amplitudes where naively divergence was expected. Understanding these enhanced cancellations through a means other than direct computation remains an open problem.

Subject: Unconventional Anderson localization in high dimensions and Dirac materials

Hamiltonians describing particles moving in a random potential often have eigenstates which have finite spatial extend, a well-known phenomenon called Anderson localization. Above two dimensional space, localized wave functions all correspond to energies below a critical energy usually called the mobility edge. The size of the localized wave functions diverges as mobility edge is approached, a phenomenon called Anderson transition. It is generally believed that the details of Anderson transition depend on the dimensionality of space only, which is usually referred to as universality of the Anderson transition. We argue that in sufficiently high dimensions a second type of Anderson transition develops if the disorder strength is close to some critical value, distinct from the conventional transition, with a number of unusual features. For a conventional Schrodinger equation with a random potential one has to be above four dimensional space to see this new transition, thus it is not straightforward, although not impossible, to observe it. In electronic systems with a Dirac-like spectrum, one only has to be above two dimensions to observe this transition. We discuss the consequences of the existence of this transition for disordered materials with Dirac-like electronic spectra.

Subject: The fate of ionizing radiation from massive stars in star-forming galaxies

The fate of ionizing radiation from massive stars has fundamental consequences on scales ranging from the physics of circumstellar disks to the ionization state of the entire universe. On galactic scales, the radiative feedback from massive stars is a major driver for the energetics and phase balance of the interstellar medium in star-forming galaxies. While even starburst galaxies appear to be largely optically thick in the Lyman continuum, ionization-parameter mapping shows that significant populations of HII regions within galaxies are optically thin, powering the diffuse, warm ionized medium. I will discuss our multi-faceted work to clarify our understanding of radiative feedback and diagnostics for probing LyC optical depth in star-forming galaxies from the Magellanic Clouds to starbursts.

In the early nineteenth century, prominent chemists, such as Jons Jacob Berzelius and Humphry Davy, proclaimed that a revolution had occurred in chemistry through electrical science. Examining Robert Hare's contributions to this discourse, this presentation analyzes how chemists understood the relationship between heat and electricity during this transformative period. As an avid experimentalist, professor of chemistry at the University of Pennsylvania, and member of the American Philosophical Society, Hare actively shaped early American chemistry. He was part of a larger network of scholars, having corresponded with a number of scientists such as Joseph Henry, François Jean Arago, and Berzelius. He published works abroad in the Philosophical Magazine in England and the Annales de Chimie in France. He also experimented with and wrote extensively on electricity and its associated chemical and thermal effects. In particular, Hare's calorimotor – a device that utilized the voltaic pile (battery) and set caloric (or heat) into motion – raised important questions about Lavoisier's caloric theory of heat and its relationship to electricity.

Subject: College in Schools: The benefits, drawbacks, and challenges of implementation for physics

This talk will compare several college-credit programs available
to high school students including CIS, AP, IB, and PSEO. I will highlight
the benefits of the CIS program and discuss the challenges to
implementation. The benefits of the CIS program include high school
students being able to take courses in their school, not having the credit
based on one test at the end of the year, and receiving credit for a UMN
course instead of just elective credits. Challenges to implementation
include teacher selection, number of teachers in a cohort, establishing
essential and non-essential components of the courses, and teacher
evaluations. The two physics courses available through the CIS program are
PHYS1101W and PSTL1163.

Subject: Measuring electrons in the solar wind: current status and future missions

Electrons are critical to the thermodynamics of the solar wind plasma. Due to their high mobility, they carry the majority of the heat flux in the solar wind. Electron beams can also be used as remote probes of the physics of shocks and solar flares. However, making accurate electron measurements is difficult: electrons are susceptible to spacecraft charging effects, and the non-thermal character of the electron distribution function limits the utility of plasma moment calculations. In this talk, I will present recent advances in precision measurements of solar wind electrons, and discuss their application to solar wind thermodynamics, shock acceleration of electrons, and electron heating associated with solar wind reconnection events. I will also discuss the measurement of electrons with particle and electric field instrumentation on the upcoming Solar Probe Plus mission.

Foodborne gastrointestinal infections are significant causes of morbidity and mortality worldwide. Alarmingly, because of the extensive, non-therapeutic use of antibiotics in agriculture, foodborne bacteria are emerging that are resistant to our most potent drugs.

We will discuss a novel approach to reduce the use of antibiotics in food-producing animals and to treat gastrointestinal infections. We engineer lactic acid bacteria (LAB) that express and release antimicrobial peptides (AMPs). LAB are part of the gastrointestinal microflora and can be safely delivered with known benefits to humans and animals. AMPs are proteins that can be readily produced by LAB. One unique aspect of our approach is the use of synthetic promoters that precisely regulate the delivery of AMP molecules.

At the heart of proposed efforts are multiscale models that guide explanations and predictions of the antagonistic activity of recombinant LAB against pathogenic strains. Models are developed to quantify how AMPs kill bacteria at distinct but tied scales. Using atomistic simulations the various interaction steps between peptides and cell membranes are explored. Mesoscopic models are developed to study ion transport and depolarization of membranes treated with AMPs. Stochastic kinetic models are developed to quantify the strength of synthetic promoters and AMPs expression. We will also present a closure scheme for chemical master equations, providing a solution to a problem that has remained open for over seventy years.1

Experimentally, we engineer lactic acid bacteria to inducibly produce antimicrobial peptides. We have used a library of synthetic biological constructs.2,3 We test these modified bacteria against pathogenic bacteria. We will present results against salmonella and enterococcus.4

A Type-I X-ray burst is the thermonuclear shell flash that happens on the surface of a neutron star. Studies on these bursts are of importance for understanding neutron stars in binary systems, nuclear reaction networks and dense matter at low temperature. I will discuss a subset of X-ray bursts that is powerful to lift up the photosphere with the simulations based on a new 1D turbulence model. The model is different in a sense that the turbulence is generated by a stochastic process and the stability of the system. The results will be compared with KEPLER's.

In the wild, microbial rhodopsin proteins convert sunlight into biochemical signals in their host organisms. Some microbial rhodopsins convert sunlight into changes in membrane voltage. We engineered a microbial rhodopsin to run in reverse: to convert changes in membrane voltage into fluorescence signals that are readily detected in a microscope. Archaerhodopsin-derived voltage-indicating proteins enable optical mapping of bioelectric phenomena with unprecedented speed and sensitivity. We are applying these tools to study the role of voltage across biology: in bacteria, plant roots, fish hearts, mouse brains, and human induced pluripotent stem cell (hiPSC)-derived neurons and cardiomyocytes. We are engineering new functionality into microbial rhodopsins by taking advantage of their strong optical nonlinearities.

For almost four decades, infrared and collinear (IRC) safety has been the guiding principle for determining which jet observables can be calculated using perturbative QCD. Now in the LHC era, new jet substructure observables have emerged which are IRC unsafe, yet still calculable using perturbative techniques. In this talk, I explain the origin of these "Sudakov safe" observables and show how they blur the boundary between perturbative and nonperturbative aspects of QCD.

In this talk I will present some of our recent theoretical efforts to understand the fascinating interplay between magnetic, charge, and superconducting order in cuprates and Fe-based superconductors. The talk will include a general discussion of spin fluctuations in iron pnictides, including the evidence and modelling of nematic (anisotropic) spin fluctuations. These can have profound influence on the transport properties of these systems both through inelastic scattering and emergent new static defect states significantly contributing to anisotropies in the measured quantities. This is true even above the magnetic transition where the anisotropic spin fluctuations can be frozen by disorder, to create elongated magnetic droplets whose anisotropy grows as the magnetic transition is approached. Such states act as strong anisotropic defect potentials that scatter with much higher probability perpendicular to their length than parallel, although the actual crystal symmetry breaking is tiny. From the calculated scattering potentials, relaxation rates, and conductivity in this region we conclude that such emergent defect states are essential for the transport anisotropy observed in experiments. I will end this part of the talk by presenting a general scenario for the transport anisotropy throughout the whole phase diagram.

Next, I will turn to a discussion of competing magnetic phases in the pnictides relevant to recent experiments finding magnetic order in a tetragonal crystal lattice. This points to the existence of other so-called C4 symmetric magnetic phases with, for example, non-collinear moments. I will present a theoretical microscopic study of these phases and their general electronic properties. A discussion will be included on the role of superconductivity and disorder in destroying and stabilizing these novel magnetic C4 states, respectively. Finally, if time allows, I will also discuss some recent studies of the doping dependence of the pairing symmetry of the cuprates in the presence of spin-density wave order.

Low-energy states of quantum spin liquids are thought to involve partons
living in a gauge-field background. We study the spectrum of Majorana
fermions of Kitaev's honeycomb model on spherical clusters. The gauge field
endows the partons with half-integer orbital angular momenta. As a consequence, the
multiplicities reflect not the point-group symmetries of the cluster, but
rather its projective symmetries, operations combining physical and gauge
transformations. The projective symmetry group of the ground state is the
double cover of the point group [1].

In recent years, there has been a push to study condensed matter systems from the point of view of high energy physics and vice versa. On the one hand, studying condensed matter systems can give high energy theorists insight about the potential nature of the Planck vacuum. On the other hand, using techniques common to high energy theorists we can make new predictions about condensed matter systems. In this talk I present my research on the topic of vortices in the B phase of superfluid helium-3. Specifically, I discuss the appearance of non-Abelian zero modes on mass vortices in helium-3-like systems.

The aurora is the visible manifestation of a electrical current system that couples the magnetosphere and ionosphere. This current system leads to the development of parallel electric fields that can accelerate the auroral particles. The "standard model" of auroral acceleration assumes that auroral particles are accelerated through a static potential drop associated with these fields. However, some auroral particle distributions do not conform to this scenario. Such distributions are thought to be accelerated through time-dependent parallel electric fields associated with Alfven waves, the analogue to waves on a string. These waves can be accompanied by parallel electric fields when the perpendicular scale size becomes small. There are two main regimes of this acceleration. At lower altitudes where the plasma is cold, electron inertial effects becomes important and can lead to the bulk acceleration of the cold plasma. At higher altitudes, the primary particle acceleration mechanism is Landau damping, which preferentially accelerates electrons with velocities near the Alfvén wave phase velocity. These mechanisms are favored in regions where there are sharp plasma gradients, such as at the plasma sheet boundary layer or on the edges of the auroral density cavity, since phase mixing is an efficient mechanism for reducing the perpendicular wavelength.

Speaker: Abena Dove Osseo-Asare, Department of History, University of Texas at Austin

Subject: From Plants to Pills: Take Bitter Roots for Malaria

Refreshments served in Room 216 Physics at 3:15 p.m.

How do plants become pharmaceuticals? In this talk, I examine the history of efforts to patent a treatment for malaria made from the bitter roots of fever vine (Cryptolepis sanguinolenta). Malaria is a serious health risk in tropical West Africa. In Ghana, where these bitter roots became known as "Ghana Quinine", a group of African scientists devoted their lives to creating a patented pharmaceutical from the plant. I consider their interactions with traditional healers from the 1940s, their struggles to establish a fledgling pharmaceutical industry, and the conflicts that complicated the success of the new drug in this postcolonial nation. This little known historical case provides a window into recent controversies surrounding biodiversity prospecting in tropical environments, the rights of indigenous peoples to shared benefits, and the quest for pharmaceutical patents. It is drawn from my recently published book, Bitter Roots: The Search for Healing Plants in Africa.

Subject: Using the Cognitive Apprenticeship Framework to Inform and Shape Instruction in Physics

Of the many types of instruction that have been used throughout human history, the apprenticeship framework is one of the most successful and enduring and its imprint can be found in a wide variety of contexts, including the coaching of elite athletes, the training of professional musicians, and the education of physics graduate students. In this talk, I will discuss the idea of a "cognitive apprenticeship," which brings ideas from the apprenticeship framework into the academic arena, as well as how it has been used to design undergraduate physics instruction at the University of Minnesota.

I will review my recent works with various collaborators on nucleosynthesis by supernovae from progenitors of different masses and how this is affected by properties and interaction of neutrinos. Implications for chemical evolution of galaxies are discussed and an initial effort to build a model for individual dwarf galaxies is presented.

Subject: Investigating the dynamics of Earth’s radiation belts through new CubeSat measurements and conjunction studies

The Van Allen radiation belts, composed of energetic ions and electrons trapped around the Earth, often exhibit dramatic variations in intensity and spatial extent. Characterization of the processes contributing to electron acceleration and loss in this region is critical to understanding the variable near-Earth space environment. Here, we’ll investigate the contribution of electron precipitation into the atmosphere to radiation belt dynamics and losses. Through a combination of long-term existing data sets as well as recent CubeSat and balloon measurements, we exploring the nature and extent of electron loss to the atmosphere as well as what electromagnetic wave modes may be causing it. These studies aid in the understanding of outer radiation belt dynamics and the relationship between precipitating energetic electrons, electromagnetic waves, and global magnetospheric conditions. They also demonstrate how small inexpensive CubeSats can complement larger missions and significantly enhance their scientific return.

Low-mass ( 8-10 solar masses) supernovae (ECSNe) which result from the collapse of O-Ne-Mg cores triggered by electron capture could provide a source of elements heavier than iron. The yields of nucleosynthesis for a 8.8 solar mass ECSN was studied based on the hydrodynamic simulation with state-of-art treatment for neutrino transport. In terms of the yield relative to iron, this nucleosynthesis is dominated by Sc and Co, and is very different from that for a regular supernova (heavier than 10 solar masses). Further, some proton-rich nuclei could be synthesized in significant amounts even in ECSN ejecta with equal number of protons and neutrons. Neutrino interactions on light nuclei are observed to smooth the yields of heavy nuclei. These results are analyzed using the theory of quasi-equilibrium nucleosynthesis. Due to their relatively clear nucleosynthetic signatures, the contributions of ECSNe could be inferred from the abundances in some extremely metal-poor stars with high [Sc/Fe] and [Co/Fe].

Subject: An asymptotic solution of large-N QCD, and of large-N n=1 SUSY YM

We find an asymptotic solution for two-, three- and multi-point correlators of local gauge-invariant operators, in a lower-spin sector of massless large-N QCD (and of n=1 SUSY YM), in terms of glueball and meson propagators, in such a way that the solution is asymptotic in the ultraviolet to renormalization-group improved perturbation theory, by means of a new purely field-theoretical technique that we call the asymptotically-free bootstrap, based on a recently-proved asymptotic structure theorem for two-point correlators. The asymptotically-free bootstrap provides as well asymptotic S-matrix amplitudes in terms of glueball and meson propagators. Remarkably, the asymptotic S-matrix depends only on the unknown particle spectrum, but not on the anomalous dimensions, as a consequence of the LSZ reduction formulae. Very many physics consequences follow, both practically and theoretically. In fact, the symptotic solution sets the strongest constraints on any actual solution of large-N QCD (and of n=1 SUSY YM), and in particular on any string solution.

Subject: High Energy" Ion depletion in the Post-Midnight Plasmasphere - Using the Van Allen Probes Satellite Suite to Uncover Mysteries of the Inner Magnetosphere

Using the Helium Oxygen Proton Electron (HOPE) instrument, Electric Fields and Waves (EFW), and the Electric and Magnetic Field Instrument Suite (EMFISIS) instruments, we examine how combining multiple instruments that characterize plasma can enable us to under how the 1-10 eV ion population, where the bulk of the plasmasphere lies, experiences strong local time asymmetry.

Subject: Are All Galaxies the Same? A Synchronized, Uniform Framework for Galaxy and Black Hole Evolution

Initial results from SPLASH, an ultra-deep multi-wavelength survey, allow a study of star formation out to z~6. Combining these results with dozens of star formation and supermassive black hole accretion studies, there is a consistent picture of galactic evolution at 0 < z < 6. These results also create tension with hierarchical merging at high redshift. We can define a "synchronization timescale" for galaxies as a measure of the uniformity of an ensemble of galaxies at various cosmic epochs. If galaxy evolution is dominated by stochastic processes, then galactic events occurring at high redshift should happen at nearly the same time across an ensemble of galaxies, while events occurring at low redshift should be much less synchronous. Surprisingly, this
synchronization timescale is both mass- and time-independent, a constant 1.4 Gyr for all combinations of mass and time. As a result, we are prompted to consider a framework for galactic evolution along a main sequence so that star formation, supermassive black hole accretion, and feedback between the two are dominated by deterministic rather than stochastic processes.

Speaker: Andrea Woody, Department of Philosophy, University of Washington

Subject: A Methodological Role for Explanation in Science: Mechanistic Explanation and the Functional Perspective

Refreshments served in Room 275 Nicholson Hall at 3:15 p.m.

Philosophy of science offers a rich lineage of analysis concerning the nature of scientific explanation. The vast majority of this work, aiming to articulate necessary and/or sufficient conditions for explanations, presumes the proper analytic focus rests at the level of individual explanations. In recent work I have been developing an alternative, which I call the functional perspective, that shifts focus away from explanations as individual achievements and towards explaining as a coordinated activity of communities.
In this talk, I outline the functional perspective and discuss certain virtues and challenges for the framework. In particular, the functional perspective suggests that explanatory discourse should be “tuned” to the epistemic and practical goals of particular scientific communities. To explore the plausibility of this contention, I examine explanatory patterns involving reaction mechanisms in organic chemistry. The aim here is to investigate ways in which such explanations are shaped to support the largely synthetic goals of the discipline. The contrast case will be mechanistic explanations in the biological sciences as recently characterized by philosophers of science. Mechanistic explanations in chemistry seem different in important respects. Most basically, I will argue that this example illustrates how taking the functional perspective may reveal an important methodological role for explanation in science, a role situated ultimately in social epistemology.

New Direct Measurement Videos allow students to vary parameters, and to select and position measurement tools. For example, a Gen-2 DMV allows students to vary the constant force on a cart to measure the resulting acceleration. Students can plot the relationship between force and acceleration. With these new tools, students can make decisions about how best to analyze the situation.

I'll show examples of how this has been implemented in my classroom, and on the MIT and BU physics MOOCs, and discuss preliminary results from last summer's AB testing in the MIT MOOC.

I'll also discuss how students can use DMVs and measurement uncertainty to explore the limits of the simplified models.

Subject: A particle physics model for inflation and the baryon asymmetry of the universe

Motivated by the coincidence between the Hubble scale during inflation and the typical see-saw neutrino mass scale, we present a supergravity model where the inflaton is identified with a linear combination of right-handed sneutrino fields. The model accommodates an inflaton potential that is flatter than quadratic chaotic inflation, resulting in a measurable but not yet ruled out tensor-to-scalar ratio. The evolution of the sneutrino fields after inflation carries a lepton charge that can be the origin of the observed baryon asymmetry of the universe. This talk is based on the preprint arXiv:1501.06560

The Earth is bathed in the atmosphere of our nearest stellar neighbor. Therefore, events occuring on the Sun's surface directly affect us by interfering with satellite operations and communications, astronaut safety, and, in extreme circumstances, power grid stability. Solar flares are a substantial source of hazardous space weather affecting our increasingly technology-dependent society and are the most energetic events in our solar system. Ground-based telescopes have been providing flare observations for over 150 years, but we now live in an era when modern space-bourne observatories provide us with even more stunning visualizations of twisted plasma escaping the Sun's surface. At the same time, these instruments give us the tools necessary to explore the physical mechanisms behind such enormous displays of energy release like never before. With nearly continuous multi-wavelength flare coverage, we can now probe the origins and evolution of flares by tracking particle acceleration, changes in ionized plasma, and the reorganization of magnetic fields. I will present some details behind flare mechanics, particularly magnetic reconnection which is a ubiquitous form of energy release throughout the cosmos, and how they affect the Earth while showing several examples of these truly fantastic explosions.

Subject: Unconventional ferrimagnetism driven by the magnetic anisotropy on the kagome lattice

The majority of ferrimagnets comprise nonequivalent spin sublattices that produce a net moment, which is an integer fraction of the magnetic moment on one site. We elaborate on an alternative mechanism that yields ferrimagnets with arbitrary net moments produced by a spin canting. This canting originates from the frustration of the spin-1/2 kagome lattice with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor couplings, as in the francisite mineral Cu_3
Bi(SeO_3)_2
Cl and its synthetic sibling compounds.

Classical treatment of the francisite spin model with only isotropic (Heisenberg) interactions leads to an infinitely degenerate ground state, where both ferrimagnetic (canted) and antiferromagnetic (spiral) phases have equal energies. Quantum corrections explored by the coupled-cluster method result in a marginal stabilization of the ferrimagnetic state. However, the main driving force behind the unconventional ferrimagnetism turns out to be the magnetic anisotropy of Dzyaloshinsky-Moriya (DM) type. By evaluating both isotropic and anisotropic (DM) terms in the spin Hamiltonian from first-principles DFT calculations, we establish a complete microscopic magnetic model of francisites and compare it to the experiment.

The opportunities for calculating isotropic exchange couplings and magnetic anisotropy parameters from DFT will be discussed.

Observations strongly indicate that chemical enrichment in nearby dwarf spheroidal galaxies (dSphs) remains inhomogeneous until the end of their star formation despite their small size. Motivated by this unsolved problem, I built a chemical evolution model for Fornax, the brightest dSph in Milky Way, based on its star formation history. I simulated stochastic and inhomogeneous mixing of newly-synthesized elements by supernovae and compared the results with the observed metallicity distributions and scatter in abundances of individual elements of e.g., Mg, Si, Ca, Ti, and Fe. This approach not only can test supernova nucleosynthesis models, but also provides insights into mixing of supernova ejecta with the interstellar medium. I found that this mixing depends on large-scale gas flows, and the differences between environments of core-collapse and Type Ia supernovae.

Most of the matter in the Universe is dark; determining the composition and interactions of this dark matter are among the defining challenges in particle physics today. I will summarize the present status of dark matter searches and the case for exploration beyond the WIMP paradigm, particularly the motivations for “light dark matter” close to but beneath the weak scale. I will also describe sharp milestones in sensitivity needed to decisively explore the best-motivated light dark matter scenarios, and comment on experimental techniques to reach these milestones.

With the discovery of the Higgs boson in 2012, the basic parameters of the Standard Model are known. At large scales of O(10^11 GeV), the quartic coupling constant of the Higgs potential becomes negative, leading to the Standard Model vacuum being metastable. Calculations of the decay rate show that the present vacuum will live much longer than the age of the Universe. I will review the features of the effective Higgs potential that lead to the instability and describe how to calculate the decay rate. I will then show that Planck scale operators, which one might naively expect to be negligible given that the instability scale is of factor of 10^8 below the Planck scale, in fact can have dramatic changes on the decay rate.

Subject: Molecular hydrogen knots in the Crab Nebula--New light on an old subject

We discovered 55 knots of molecular hydrogen in the Crab Nebula. What does this new component tell us? The H2 knots are associated with a cooler population of the well-known filaments, and strangely 80% are on the back side of the nebula. The temperature of the H2 emission is 2500-3000K. We have images and spectra from the SOAR Telescope and images from HST; we have new spectra from the Large Binocular Telescope.

I will begin with a brief review of Mott physics in the one-band case. After having introduced the necessary terminology I introduce a general multi-band model and argue that, due to the presence of the Hund's coupling in such a model, the system can undergo a so-called orbital-selective Mott transition [1]. Subsequently, I highlight the substantial differences between the half-filled case and the non-half-filled case [2]. Finally I will discuss the possible manifestation of this transition in the iron-chalcogenides [3].
[1] L. de' Medici et al. Phys. Rev. Lett. 102, 126401 (2009).
[2] L. de' Medici, Phys. Rev. B. 83, 205112 (2011).
[3] Z. Xu et al. Phys. Rev. B. 84 052506 (2011).

Subject: Recent findings for the Seebeck effect in thermoelectric tellurides

The Seebeck effect, known since 1821 and utilizing in thermoelectric materials to convert thermal to electrical energy, have been intensively studied and used in various applications, but there are indications that its fundamental understanding is still lacking. In order to better understand the Seebeck effect, we have used tellurides as model systems, common experimental methods (XRD, Seebeck coefficient, electrical resistivity, Hall effect, thermal conductivity, SEM, and EDS), and advanced method (125Te NMR to obtain the carrier concentration in tellurides via spin-lattice relaxation measurements). Several intriguing relations between the Seebeck coefficient, carrier concentration, composition, and rhombohedral lattice distortion have been established for GeTe alloyed with Ag and Sb (well-known high thermoelectric efficiency TAGS-m series). The Seebeck coefficient in some tellurides can be described by a common model with energy independent carrier scattering, but in some not. The latter can be explained by the effect from additional mechanism, likely energy filtering produced by [Ag+Sb] pairs in GeTe matrix. Our findings demonstrate that the Seebeck effect still holds surprises, and innovative research in the area of thermoelectric materials may help to elucidate and better utilize thermoelectric phenomena.

I will discuss a recent paper on ALMA observations of 3 high z galaxies, 6.8 < z < 7.1, searching for [CII] emission. Detected [CII] emission is interpreted as arising from accreting clumps of neutral gas, showing galactic assembly at z~7. I will discuss their observations, results, and simulations.

Electronic systems can have a type of order in which coherence is spontaneously established between two distinct groups of electrons. So far this (particle-hole or exciton condensate) type of order has been found only in double-layer two-dimensional electron gas systems, and only in certain strong magnetic field limits. I will review some of the surprising superfluid transport effects that have been observed in double-layer exciton condensates, and speculate on the possibility of realizing similar effects at room temperature either by enhancing the stability of bilayer exciton condensate states or by designing ferromagnetic materials with appropriate properties.

Subject: New Constraints on Cosmic Reionization from Planck and Hubble Space Telescope

Understanding cosmic reionization requires the identification and characterization of early sources of hydrogen-ionizing photons. The 2012 Hubble Ultra Deep Field (UDF12) campaign has acquired the deepest blank-field infrared images with the Wide Field Camera 3 aboard Hubble Space Telescope and, for the first time, systematically explored the galaxy population deep into the era when cosmic microwave background (CMB) data indicates reionization was underway. The UDF12 campaign thus provides the best constraints to date on the abundance, luminosity distribution, and spectral properties of early star-forming galaxies. We synthesize the UDF12 results with results from other HST campaigns and the most recent constraints from Planck CMB observations to infer redshift-dependent ultraviolet luminosity densities, reionization histories, and electron scattering optical depth evolution consistent with the available data. We review these results, and discuss future avenues for progress in understanding the epoch of reionization.

In the 1970s, Nigeria's oil boom generated unprecedented state wealth, quite in contrast to a massive U.S. economic recession. During that period, U.S. and European multinational companies turned to Nigeria to manufacture drugs and sold them on what was then a significant and important foreign market in terms of sales. By the 1990s, brand name drug markets in Nigeria and throughout Africa were completely eviscerated and relocated elsewhere. What was once almost exclusively a brand name drug market is now home to mostly imported pharmaceuticals throughout the world, for which there are constant concerns over drug quality. The paper first discusses two simultaneous convergences that remade the West African brand name market: Nigeria's structural adjustment program and the pharmaceutical industry's turn to speculative capital. It then provides an overview of the kinds of markets and the kinds of drugs that emerged in the aftermath of brand name industry's abandonment of the West African market. It concludes with a discussion on how actors within Nigerian and global drug markets interact with chronic, and indeed anticipated, market volatility in ways that produce new orders of pharmaceutical value.

Subject: Reimagining the Physics TA: A Deeper Look Into the Essential Role of Coach

In this seminar, I will discuss the role of a TA in the Minnesota Model for teaching physics from my perspective as both a TA for the intro physics courses and a mentor to the new graduate and undergraduate TAs. In addition, this session will explore the learning theories which connect to the coaching methods used by TAs, contrasting these methods with more traditional ones, and highlight the opportunities that the structure of our discussions/labs creates for our students. Finally, after noting the pedagogical benefits of the ideal form of the model and the importance of the TA in facilitating the learning process, we will discuss the challenges that physics TAs in our department often face, and explore possible remedies.

M82, NGC 253, and Arp 220 are often associated with each other due to similarities in the intense starburst environments contained within each galaxy. Dense concentrations of young massive stars, strong magnetic fields, and high radiation fields characterize their starburst nuclei. Additionally, both M82 and NGC 253 have been detected in gamma-rays with Fermi. Despite their similarities, the interstellar medium and effects of galactic winds differ in these galaxies. These distinctions are vital to understanding the cosmic ray populations and how their interactions produce the observed radio and gamma-ray spectra from each galaxy. I will discuss results of my single-zone models of the cosmic ray populations of the starburst nuclei and their implications for future gamma-ray and neutrino observations.

Fractionalization is a property of both topological phases and spin liquids. Moreover, in many cases it may be the easiest characteristic to use to identify these systems in experiments. I will talk about recent work in classifying the way that symmetry acts on fractional quasiparticles [1]. I will introduce the classification scheme and discuss the measurable signatures in relation to exactly solvable examples such as Kitaev’s Toric Code model [2]. (Recent applications are [3] and [4]).
[1] “Classifying fractionalization,” Essin and Hermele, PRB 87, 104406 (2013)
[2] “Spectroscopic signatures of crystal momentum fractionalization,” Essin and Hermele, PRB 90, 121102(R) (2014)
[3] “Numerical detection of symmetry-enriched topological phases with space-group symmetry,“ Wang, Essin, Hermele, and Motrunich: PRB 91, 121103(R) (2015)
[4] “Detecting crystal symmetry fractionalization from the ground state: Application to Z2 spin liquids on the kagome lattice,“ Qi and Fu, PRB 91, 100401(R) (2015)

Understanding the origin of the short-lived radio isotopes (SLR) that were present in the early solar system (ESS) is crucial in understanding the events leading to the formation of the Solar System. Many of the SLR require a nucleosynthetic event within about a million years before the formation of the Solar System. We show that low mass compact progenitors of
can self-consistently account of some of the very short-lived isotopes such as ^{41}
Ca, ^{53}
Mn, and ^{60}
Fe. Furthermore, neutrino-induced spallation can also account for ^{10}
Be in the ESS, which until now were thought to be produced only by cosmic ray irradiation.

Superconductors may be grouped into two major classes. The first is conventional metallic, whose pairing mechanism is explained by the BCS theory and electron-phonon coupling. The second we call unconventional, and the precise pairing mechanism has still to be worked out. All of the unconventional superconductors have electronic properties that are highly tunable, either by doping or pressure, from a non-superconducting parent compound, to a superconductor, to a non-superconducting Fermi liquid, thus defining a superconducting ‘dome’ in the phase diagram. More than 40 families of such materials, including the high-temperature superconductors, exhibit this ubiquitous phase diagram. In the underdoped phases, all of these materials show intriguing correlated electron states above the dome, and researches agree that the understanding of this “electron matter” holds the key to the pairing mechanism. Finally, I will show how we have found that point contact spectroscopy is exquisitely sensitive detecting electron matter.

I will discuss the mechanism of accidental SUSY, the notion of having non-supersymmetric RG flows ending on supersymmetric fixed points, from the point of view of QFT, RS and string theory. I will also discuss the mechanism's utility in BSM model building, specifically as a way of UV-completing natural SUSY in the model of "resurgent SUSY", solving both the little and big hierarchy problems. I will argue that a Z2 orbifold of the Klebanov-Strassler solution exhibits accidental SUSY, and furthermore is a prototype for the strong sector that should be present in resurgent SUSY.

The physics of quantum critical phase transitions connects to some of the most difficult problems in condensed matter physics, including metal-insulator transitions, frustrated magnetism and high temperature superconductivity.
Near a quantum critical point(QCP)a new kind of metal emerges, whose thermodynamic and transport properties do not fit into the unified phenomenology with which we understand conventional metals - the
Landau Fermi liquid(FL)theory - characterized by a low temperature limiting T-linear specific heat and a T^2 resistivity.Studying the evolution of the T­ dependence of these observables as a function of a control parameter leads to the identification both of the presence and the nature
of the quantum phase transition in candidate systems. In this study we measure the transport properties of BaFe2(As1-xPx)2,at T
by suppressing superconductivity with high magnetic fields. We find an anomalous magnetic field dependence that suggests that not only does magnetic field directly affect the scattering rate (which is unusual for metals), but it does so in a way that is identical to temperature. We suggest that there is a universal phenomenology of scattering near a quantum critical point.

We provide a solution for the supersymmetric CP(N-1) at large N
written in superfields. Specifically, we find the Kahler potential of
the model by supersymmetrizing the known results for the effective
potential of the theory. We also generalize the problem by introducing
twisted masses, and by breaking supersymmetry to N=(0,2). Such
theories arise as effective models on the heterotic vortex strings.

Subject: Rural Health and Striking Urban Doctors: The Aftermath of Mexico's Attempt to Provide Healthcare for All

Refreshments served in Room 216 Physics at 3:15 p.m.

In late 1964 residents and interns walked out of Mexico City hospitals and clinics in what would become a ten-month-long medical movement. In the months that followed the walkout, young doctors' demands shifted from salary and better working conditions to openly questioning an increasingly oppressive regime. Mexico had long been seen as a leader in health care delivery in Latin America and the image of doctors protesting in the streets became a visible sign of the failings of the nation's public health care system. This presentation examines the unlikely aftermath of the medical movement as well as its causes and links to earlier policies to provide healthcare to rural Mexicans.

In this talk I will present results on a model of spatial evolution on a lattice, motivated by the process of carcinogenesis from healthy epithelial tissue. Cancer often arises through a sequence of genetic alterations. Each of these alterations may confer a fitness advantage to the cell, resulting in a clonal expansion. To model this we will consider a generalization of the biased voter process which incorporates successive mutations modulating fitness, which is interpreted as the bias in the classical process. Under this model we will investigate questions regarding the rate of spread and accumulation of mutations, and study the dynamics of spatial heterogeneity in these evolving populations.

The concept of ‘Majorana fermions’, first proposed by Ettore Majorana that spin-1/2 particles could be their own antiparticles, is ubiquitous in modern physics nowadays. Majorana’s big idea was supposed to be solely relevant to high energy physics like neutrino, dark matter and supersymmetry. But now, his conjecture can not only be fulfilled in solid-state systems but also bring a promising future to the quantum information processing. In this talk, I will first introduce the historical background of Majorana fermions. Then I will bring the Majorana physics to solid-state systems by introducing two toy models that support Majorana modes. First is the Kitaev’s toy lattice model for 1D p-wave superconductor and the second is Fu-Kane model for 2D p+ip superconductor. Finally I will discussion the non-Abelian exchange statistics associated with the Majorana modes and their potential for quantum information processing.
Reference:
[1] J. Alicea, Reports on Progress in Physics, vol. 75, p. 076501, 2012.
[2] S. R. Elliott and M. Franz, Reviews of Modern Physics, vol. 87, pp. 137-163, 02/11/2015.
[3] A. Y. Kitaev, Physics-Uspekhi, vol. 44, p. 131, 2001.
[4] J. Alicea, Y. Oreg, G. Refael, F. von Oppen, and M. P. Fisher, Nature Physics, vol. 7, pp. 412-417, 2011.

Intermediate phases with “vestigial order” occur when the spontaneously broken symmetries of a “fully ordered” groundstate are restored sequentially as a function of increasingly strong thermal or quantum fluctuations, or of increasing magnitude of quenched randomness. From this perspective, a large number of developments in the field of highly correlated systems – in particular the remarkable proliferation of cases in which electron nematic phases and their relatives appear as significant players in the physics of “interesting” materials - can be treated as variations on the same underlying theme. Recent experiments probing charge order in the pseudo-gap regime of the hole-doped cuprate high-temperature superconductors and nematic order in the Fe based superconductors are interpreted in light of these results.

I will discuss several aspects of modern treatments of the on-shell S-matrix. In particular, I will give an inductive proof of on-shell recursion relations --- due to Britto, Cachazo, Feng, and Witten (BCFW) --- to graviton amplitudes in four dimensions. Unlike previous proofs of applicability in the literature, this proof relies exclusively on on-shell reasoning. As a corollary of this analysis, we show that the formerly mysterious ``bonus'' scaling of graviton amplitudes under large BCFW-shifts follows simply from Bose symmetry. I will also discuss a way to extract many beautiful, classic, no-go theorems from straightforward analysis of four-point scattering amplitudes.

Subject: Enhancement of superconductivity near a nematic quantum critical point

Please note: change of time for seminar, in effect for rest of semester.

Two topics that have attracted intense theoretical study over the past decade are the nature of quantum critical phenomena in metallic systems and what, if anything, such critical points have to do with an unconventional mechanism of superconducting pairing. The still un-mastered subtleties of the first problem have precluded convincing resolution of the second. For the model problem of a weakly interacting metal in proximity to a nematic quantum critical point (NQCP), we identify a broad regime of parameters in which the nature of the induced superconductivity can be understood in a theoretically well controlled manner without needing to resolve the deep, unsolved issues of metallic criticality. We show that: 1) a BCS-Eliashberg treatment remains valid outside of a parametrically narrow interval about the NQCP; 2) the symmetry of the superconducting state (d-wave, s-wave, p-wave) is typically determined by the non-critical interactions, but Tc is enhanced by the nematic fluctuations in all channels; 3) in 2D, this enhancement grows upon approach to criticality up to the point at which the weak coupling approach breaks-down, but in 3D the enhancement is much weaker. Preliminary results of determinental quantum Monte-Carlo studies of the true critical regime will also be presented.

While astronomers generally agree that Type Ia supernovae mark the complete disruption of a white dwarf star, the mechanism for destabilizing the white dwarf and driving it to this suicidal end remains a mystery. I will describe the current state of efforts to determine the progenitor systems of Type Ia supernovae, and detail our team's efforts to narrow the possibilities with sensitive measurements of the environments of nearby supernovae. Finally, I will compare what we know about Type Ia supernovae with recent observations of Milky Way binary stars, testing whether the white dwarfs in these real-world binaries may one day meet the violent end of Type Ia supernovae.

Speaker: Mazviita Chirimuuta, Department of History and Philosophy of Science, University of Pittsburgh

Subject: Ontology of Colour, Naturalized

Refreshments served in Room 275 Nicholson Hall at 3:15 p.m.

Can there be a naturalized metaphysics of colour—a straightforward distillation of the ontological commitments of the sciences of colour? In this talk I first make some observations about the kinds of philosophical theses that bubble to the surface of perceptual science. Due to a lack of consensus, a colour ontology cannot simply be read off from scientists' definitions and theoretical statements.

I next consider three alternative routes towards a naturalized colour metaphysics.
1) Ontological pluralism—endorsing the spectrum of views associated with the different branches of colour science.
2) Looking for a deeper scientific consensus.
3) Applying ideas about emergent properties that have been useful elsewhere in biology.

Subject: The gap: Where faculty perceptions of teaching and learning disconnect from practice.

Faculty perceptions of teaching and learning and the functionality of those perceptions in the classroom have been studied for over 20 years now. Understanding of these ideas has led to curriculum changes that better engage students and employ faculty understanding. These same studies have shown that there is a gap in implementation of these curriculum that can effect student outcomes. This talk will be an overview of past studies and discuss where this gap may lie. Next step research proposals will also be discussed.

Basis Light Front Quantizition (BLFQ) is a nonperturbative treatment of quantum field theory, which adopts light-front quantization and the Hamiltonian formalism. BLFQ offers a first principles solution to many outstanding puzzles in nuclear and particle physics. BLFQ can also be used to study time-dependent scattering processes (tBLFQ). As its first realistic application, we apply principles of tBLFQ to lepton pair production in ultraperipheral heavy ion collisions. Due to the strong electromagnetic fields generated in heavy ion collision, perturbative approaches may lose its prediction power especially if two heavy ions collide with small impact parameter. As a non perturbative approach, tBLFQ takes into account the production and multiple-scattering of the leptons by the strong electromagnetic fields generated in the collisions inherently. Preliminary results with highly truncated basis will be presented.

Dr. Schuhmann is the Managing Editor of Physics Review Letters. Physical Review Letters receives ~11000 submissions per year, and publishes about 1/4 of them. Editors decide what to publish with extensive input from peer review, with roughly 70% of manuscripts reviewed. My talk will outline of how PRL manages peer review for such volume, with examples from correspondence, while it remains the premier physics journal. It is the most cited physics journal, with a Letter cited roughly every 90 seconds. PRL faces many challenges, however, as the publishing trends in some areas of physics shift, for example to smaller, less comprehensive, or more interdisciplinary venues. I will discuss some of these challenges, and what PRL is doing, and plans to do, to maintain a competitive journal that covers the full arc of physics. I would greatly appreciate your feedback and questions during and after the talk.

Subject: A Particle Physicist's Nightmare: What If Dark Matter is Truly Dark?

Despite being ubiquitous throughout the Universe, the fundamental physics governing dark matter remains a mystery. Current dark matter searches implicitly assume that dark matter couples at some level to ordinary baryonic matter via Standard Model interactions. However, it remains possible that dark matter interacts so weakly with ordinary matter that it will escape detection with current and near-future technology. This particle physicist's nightmare forces us to consider other possible probes of dark matter physics. For instance, the evolution of cosmological density fluctuations on small causal length scales in the early epochs of the Universe is a very sensitive probe of the fundamental physics of dark matter. Studying the astrophysical structures that resulted from the gravitational collapse of fluctuations on these small scales can thus yield important clues about physical processes that took place in the dark sector in the early Universe. Today, most of these structures are locked-in deep inside the potential wells of massive galaxies, making the study of their properties difficult. Fortunately, due to fortuitous alignments between high-redshift bright sources and us, some of these galaxies act as spectacular strong gravitational lenses, allowing us to probe their inner structures. In this talk, we present a new statistical analysis formalism to extract information about mass substructures inside lens galaxies and discuss what this can tell us about dark matter physics.

Please note: change of time for seminar, in effect for rest of semester.

Iridium oxides are predicted to host a variety of exotic electronic phases emerging from the interplay of strong electron correlations and spin-orbit coupling. There is particular interest in the perovskite iridate Sr2IrO4 owing to its striking structural and electronic similarities to the parent compound of high-Tc cuprates La2CuO4. However, despite theoretical predictions for unconventional superconductivity and recent observations of Fermi arcs with a pseudogap behavior in doped Sr2IrO4, no superconductivity has been observed in this compound so far. In this talk I will describe the nonlinear optical spectroscopy and wide field microscopy techniques that we have recently developed to resolve the symmetries of both lattice and electronic multipolar ordered phases on single domain length scales. I will show our results on the undoped Mott insulator Sr2IrO4 that reveal a subtle structural distortion and provide evidence for a hidden loop-current ordered magnetic phase that has previously eluded other experimental probes. The significance of these novel orders to magneto-elastic coupling and the pseudogap phase in Sr2IrO4 will be discussed.

Speaker: Francesca Bray, Social Anthropology, University of California Santa Barbara/University of Edinburgh

Subject: Happy Endings: Narratives of Reproduction in Late Imperial China

Refreshments served in Room 216 Physics at 3:15 p.m.

A rich resource for exploring the reproductive cultures of late imperial China ca. 1500 – 1800 is the abundant corpus of gynecology (fuke) treatises and case-histories. Demographic historians have recently used quantitative sources to argue that deliberate checks on fertility became common during this period, and that a rational, "modern" demographic mentality emerged which saw elite or better-off families matching numbers of children to resources and opportunities. In documenting specific attempts to intervene in natural processes, the fuke medical cases offer some very different perspectives on how childbirth and fertility were understood by Chinese families, what was considered a successful outcome, what a failure, and whose opinions counted. Here I focus on the temporal framing and narrative choices of selected fuke cases to ask what they can tell us about how practitioners and their clients attempted to control reproductive processes, and about the ideals, decisions and emotions associated with childbearing. The medical sources corroborate several elements of the demographers' model of reproductive agency and rationality, yet vividly portray the uncertainty, peril and intense emotions of reproductive life, and underline the heavy price the many women had to pay in order to produce a socially desirable family.

Subject: Advances in Biodetection with Optical and Mechanical Microresonators

Dr. Frank Vollmer will present his results on advancing chip-scale
biosensing capabilities with optical and mechanical microresonators.
In the optical domain, he has developed a microcavity biosensing
platform that is capable of monitoring single DNA molecules and
their interaction kinetics, hence achieving an unprecedented
sensitivity for label-free detection with light. In the mechanical
domain, he is developing a new force-based biosensing technique
based on the quartz crystal microbalance. By applying centrifugal
forces to a sample, it is possible to repeatedly and
non-destructively interrogate its mechanical properties in situ and
in real time.

The coexistence of different ordered electronic states in metals has been discussed a lot. In iron pnictides, superconducting and magnetic spin density wave orders influence each other, e.g. they can support each other and lead to coexistence states [1]. This coexistence depends on the shape and area of the Fermi surface. In my presentation, I will briefly review the concepts of Fermi surface nesting and spin density waves. Then, a Ginzburg-Landau analysis in the vicinity of the crossing point of SC and SDW transitions and at T=0 will be discussed [2]. After that, I will present some numerical results and compare with the Ginzburg-Landau theory.
[1]Chubukov, Andrey V., D. V. Efremov, and Ilya Eremin. "Magnetism, superconductivity, and pairing symmetry in iron-based superconductors."Physical Review B 78.13 (2008): 134512.
[2] Vorontsov, A. B., M. G. Vavilov, and A. V. Chubukov, Physical Review B 81.17 (2010): 174538.

Refreshments served in Tate Foyer after colloquium. This is the more technical portion of the Van Vleck Lecture series. This lecture is free and open to the public.

Clusters of galaxies are still forming at the current epoch. At the turnaround radius, the infall velocity induced by the mass concentration in the cluster just balances the Hubble flow. In the standard cosmology, galaxies within the turnaround radius will remain bound to the cluster and the mass within this radius is a good estimator of the ultimate cluster mass. Our redshift survey of nearly 100 massive clusters (HeCS: Hectospec Cluster Survey) in the redshift range 0.1 to 0.3 enables a number of important cosmological probes. These include a test of the cluster mass proxy derived from observations of the Sunyaev-Zeldovich effect. These data also enable measurement of mass profiles to large radius using the caustic technique. These profiles enable direct measurement of the accretion rate of galaxy clusters and of their ultimate mass. These measurements are new and direct tests of our understanding
of the growth of structure in the universe.

We discuss a class of fermionic dark matter candidates which are charged under
the SU(2)_L gauge interactions, and evaluate their scattering cross section
with a nucleon, which is an important quantity for dark matter direct
detection experiments. Such a fermionic dark matter particle interacts with
quarks and gluons through one- and two-loop processes, respectively.
We will find that the resultant scattering cross sections lie around O(10^{-(46-48)}) cm^2, which exceed the neutrino background and thus can be reached in
future direct detection experiments.

This is event is free and open to the public. It is scheduled to last an hour.

We live in the first time when it is possible to map the universe. We now know that galaxies like our own Milky Way trace the largest patterns in nature. These patterns, first uncovered by the CfA redshift survey in 1986-1989, extend for hundreds of millions of light years. Now with large telescopes, we can trace the evolution of these patterns and compare them with some of the worlds largest computer simulations. A new survey, HectoMAP shows us the patterns in the universe nearly 7 billion years ago. A HectoMAP movie takes us through the data. Comparison of HectoMAP observations with simulations provides new direct tests of our understanding of the evolution of structure in the universe.

Subject: From Filaments, to Core, to...Filaments?! The Role of Magnetic Fields in Multi-Scale, Filamentary Star Formation

In just the past few years, it has become clear that filamentary structure is present in the star-formation process across many orders of magnitude in spatial scale, from the galactic scales probed by Planck and Herschel all the way down to the AU-scale structures that ALMA has revealed within protoplanetary disks. A similar story can be told of magnetic fields, which play a role in star formation across the same vast range of size scales. Here I will first review my work on 1000 AU-scale dust polarization and magnetic fields in Class 0 protostellar envelopes, which were observed as part of the TADPOL survey using the 1.3 millimeter dual-polarization receiver system at CARMA. I will then highlight two 1000 AU-scale filamentary structures seen with CARMA before I reveal new, high resolution (150 AU!) ALMA 1.3 mm continuum observations of three protostars in Serpens. Even at such high resolution, these sources have a number of nearby, filamentary blobs/condensations/ companions, most of which coincide in a tantalizing way with the magnetic fields we mapped with CARMA. I will muse on what this all means, and on what questions may soon be answered by ALMA polarization observations of the same three sources.

Speaker: Leslie Tomory, History and Classical Studies, McGill University

Subject: London's Water Supply before 1800 and the Origins of Network Modernity

Refreshments served in Room 216 Physics at 3:15 p.m.

Since the middle of the nineteenth century, integrated technological networks have proliferated, especially in the Western world. This talk argues that one of the roots of this technologically networked society is London's water supply network. This infrastructure network was first founded in 1580, and by the 18th century, tens of thousands of houses were connected to it. The builders of this network solved a host of business, financial, legal and technological problems, and in doing so, created a model that was explicitly used by later network builders.

The pairing mechanism in high temperature cuprate superconductors is an interesting problem. A low energy effective theory called spin-fermion model can be applied to study cuprates, and it's argued that pairing in cuprates is mediated by spin fluctuations. The model describes low-energy fermions interacting with their own collective spin fluctuations.

For phonon mediated superconductors, vertex corrections of phonon electron interactions and momentum dependence of fermionic self energy are small and neglected, due to the small ratio of sound velocity and Fermi velocity. This leads to the well known Eliashberg equations for superconducting state. For cuprates, Eliashberg-type theory is still valid, but for different reasons with phonon mediated superconductors, in spite of strong spin fermion interactions. Many 'fingerprints' of the spin fluctuation mediated pairing mechanism has been seen in the experiments.

This is the first talk of two consecutive Journal Club talks using spin-fermion model to study cuprates. I will focus on:
1.Introduction to spin-fermion model.
2.Summary of Eliashberg theory for electron-phonon pairing.
3.Perturbation theory and Eliashberg-type equations for spin fermion model.
4.Two of the fingerprints: Comparison with ARPES and neutron scattering experiments for superconducting state.

This talk will describe the calculations of iso-vector and iso-scalar nucleon charges g_A, g_S and g_T. These quantities are needed to probe new physics in precision experiments of neutron decay and electric dipole moment nEDM. Discussion of lattice QCD results will address systematic uncertainties associated with the lattice volume, lattice spacing, quark mass and renormalization of the novel CP violating operators. I will provide results for the quark electric dipole moment contribution to nEDM and the status of the calculations of the quark chromoelectric dipole moment being calculated by the LANL-RBC-UKQCD collaboration.

The axion is a hypothetical elementary particle whose existence would explain the baffling absence of CP violation in the strong interactions. Axions also happen to be a compelling dark-matter candidate. Even if dark-matter axions were to comprise the overwhelming majority of mass in the universe, they would be extraordinarily difficult to detect. However, several experiments, either under construction or taking data, would be sensitive to even the more pessimistically coupled axions. This talk describes the current state of these searches.

I will begin with a brief and qualitative (but self-contained) overview of the semiclassically calculable small-S^1 confining dynamics in deformed Yang-Mills theory and QCD(adj) (including N=1 super Yang-Mills). I will then study confining strings in these theories, contrasting their properties with those of similar theories exhibiting abelian and nonabelian confining regimes, and will discuss unsolved problems for the future.

Polarized radiation at optical through millimeter wavelengths arises from thermally emitting dust grains whose axes exhibit a net alignment with the local magnetic field. As a result of this mechanism, polarization observations have successfully provided one of the few methods for measuring astrophysical magnetic field strengths and structure in a wide range of objects, from young protostars to external galaxies. The physical mechanisms proposed for aligning the grains have met some, but not every, observational test (i.e., correlations between polarization and reddening, spectral dependence, and grain composition). In the absence of a well-understood alignment model, the aforementioned magnetic field measurements would all be suspect.

Here I will introduce some key predictions from grain alignment models and discuss the observational successes and failures of each, leading to a preferred model in which interstellar radiation drives grain alignment. Additional tests of this radiative alignment torque model should be possible with new facilities such as ALMA, HAWC/SOFIA, and recently released Planck polarization data.

Speaker: Erik Angner, Department of Philosophy, George Mason University

Subject: There Is No Problem of Interpersonal Comparisons

Refreshments served in Room 275 Nicholson Hall at 3:15 p.m.

The proposition that interpersonal comparisons of utility are impossible has been part and parcel of mainstream economics for almost a century. These days, the proposition is invoked inter alia in arguments against happiness-based measures of well-being, which average happiness scores across populations in an effort to represent social welfare. In this talk, I will argue that interpersonal comparisons of utility are in fact implicit in virtually all traditional economic social welfare measures as well; if such comparisons are problematic, then, the problem is not unique to happiness-based measures. Fortunately, however, I will argue but that the proposition is a piece of zombie methodology: a methodological prescription that should have been dead and buried a long time ago. Social welfare measures have many problems, but interpersonal comparisons isn't one.

The Soudan Underground Physics Lab is holding an open house on Saturday, May 2, starting at 8:30 a.m. and continuing throughout the day. Visitors can travel 1/2-mile underground and discover the data collected during the years that the MINOS detector has been operating. The Fermi National Accelerator Lab has been sending a beam of neutrinos to the Soudan Lab since the first one was detected on March 20, 2005.

The simulation of quantum many-body systems on classical computers is notoriously difficult because of the exponential growth of the Hilbert space with the size of the system. This makes the study of some of the most fascinating problems in condensed matter physics extremely challenging. These include for example the understanding of high TC superconductors, as well as frustrated quantum magnets which might realize exotic phases of matter. In my talk, I will discuss how numerical approaches based on quantum information concepts allow for an efficient simulation of quantum many-body systems. In particular, I will introduce tensor-product state based methods that provide an optimized representation of the relevant corner of the Hilbert space. I will then demonstrate applications of matrix-product state based methods to obtain the ground state as well as the dynamics of strongly correlated quantum systems.

A new class of solutions to the electroweak hierarchy problem is presented that does not require either weak scale dynamics or anthropics. Dynamical evolution during the early universe drives the Higgs mass to a value much smaller than the cutoff. The simplest model has the particle content of the standard model plus a QCD axion and an inflation sector. The highest cutoff achieved in any technically natural model is 10^8 GeV

Subject: Frustration, Unexpected Order and Criticality in 2D Heisenberg Antiferromagnets

Please note: change of time for seminar, in effect for rest of semester.

Despite having finite spin correlation lengths at nonzero temperatures, frustrated two-dimensional Heisenberg magnets can develop long-range discrete order driven by short-range thermal spin fluctuations. Indeed this "order from disorder" phenomenon can lead to a finite-temperature Ising or Potts transition; it has recently been realized experimentally in the iron-based superconductors where it is responsible for the high-temperature nematic phase. We can also ask whether such a mechanism can lead to an emergent critical phase in an isotropic Heisenbergm magnet and we present a model where this is indeed the case. Our results are supported by both Wilson-Polyakov scaling and by Friedan's geometrical approach to nonlinear sigma models. We also discuss recent computational studies that support our results and discuss possible experimental realizations. We end with a more general discussion of the relation between Friedan scaling and 2D antiferromagnetism, and the possibility of using it to simulate generalized surgery-free Ricci flows of topological manifolds of broader mathematical interest.
Work done in collaboration with P. Orth, R. Fernandes, B. Jeevanesan, J. Schmalian, P. Coleman and A.I. Larkin.

Subject: "Shining a Light Into the Dark: Using Cave Deposits to Illuminate Fine-Scale of the Geomagnetic Behavior

Cave deposits, such as stalagmites and stalactites, record the direction
and strength of the Earth's magnetic field as they grow. These geologic
materials offer a new source of information about the behavior of the
Earth's geodynamo. Speleothems record their magnetizations on seasonal
timescales, they are are not affected by the post-depositional processes
that effect marine and lake records, and can be dated with high precision using U-Th techniques. Recent improvements in the sensitivity of magnetic instrumentation and spatial resolution allow geophysicists to leverage speleothems as high-resolution paleomagnetic recorders. Modern studies enable us to resolve short-term geomagnetic variability, and characterize events such as geomagnetic reversals and excursions at an unprecedented scale.

In this talk I discuss the material and cultural manufacture of the modern dog. By 'modern dog', I mean an animal seen principally in terms of its 'breed'; that is, a specific physical conformation and related behavioural characteristics. Its inventors were British middle and upper class aficionados of dog shows, which were events of sporting competition, commercial speculation and sociality. They grew in popularity from the 1860s and by 1900 had spread, with their new types of canine, across the world. The ways in which dog shows were organised encouraged, and then required, dog breeders to reshape the existing variety of dog types into standardised forms called breeds, and to record the breed history of dogs in pedigree. The very first modern dog was a pointer named 'Major', so defined in 1865 by John Henry Walsh (aka 'Stonehenge'). He became the model for all subsequent members of the breed; however, in the spirit of the times, the 'improvement' was expected through breeding with and for good blood.

In a static topological system the Chern number is equal to the number of chiral edge modes. However, in a periodically driven system there may be chiral edge modes present even in the case of trivial Chern number for all bands. In this talk I will use a toy model and Floquet theory to demonstrate this possibility, even if the Floquet operator is trivial i.e. the bulk ground state doesn't change under one period of evolution. Then I will show the derivation of a generalization to the Chern number that gives the number of edge states in a periodically driven topological system. In the end I will discuss the effects on a more realistic system with weak driving, as it may be realized in cold atom gases.
Reference:
Mark Rudner, Netanel Lindner, Erez Berg, and Michael Levin: PRX 3, 031005 (2013)

Please note: change of time for seminar, in effect for rest of semester.

Frustrated magnetism has become an extremely active field of research. The concept of geometrical frustration dates back to Wannier’s 1950 study of Ising antiferromagnet on the triangular lattice. This simple system illustrates many defining characteristics of a highly frustrated magnet, including a macroscopic ground-state degeneracy and the appearance of power-law correlations without criticality. In this talk I will discuss a simple generalization of the triangular Ising model, namely, a finite number of vertically stacked triangular layers. Our extensive numerical simulations reveal a low temperature reentrance of two Berezinskii-Kosterlitz-Thouless transitions. In particular, I will discuss how short-distance spin-spin correlations can be enhanced by thermal fluctuations, a phenomenon we termed stiffness from disorder. This is a generalization of the well-known order-by-disorder mechanism in frustrated systems. I will also present an effective field theory that quantitatively describes the low-temperature physics of the multilayer triangular Ising antiferromagnet.

It is well known that spontaneous symmetry breaking can only take place in the thermodynamic limit. Yet, by general hydrodynamic and effective field theory arguments, fingerprints of this symmetry breaking are already encoded in finite volume system realizations. Here, I will discuss how to dig out these fingerprints in exact low-energy spectra of finite-size systems, a method which has been extremely successful over the last 20 years (starting e.g. with the first ‘numerical proof’ of long-range magnetic ordering in the 2D triangular antiferromagnet). The power of this method is not about system-size extrapolations (‘large enough’ system sizes are anyway quickly unreachable for many strongly correlated systems), but on the full exploitation of symmetry, which gives very stringent predictions for the structure and the exact content of the low-energy spectra. The underlying principles offer one of the most direct ways to understand the mechanism of spontaneous symmetry breaking, and as such it is a general and fundamental topic that goes beyond numerical simulations.
References:
[1] P. W. Anderson, PRB 86, 694 (1952)
[2] C. Lhuillier, cond-mat/0502464v1, chapter 2
[3] B. Bernu et al, PRL 69, 2590 (1992); PRB 50, 10048 (1994)
[4] P. Azaria et al, PRL 70, 2483 (1993)
[5] H. Neuberger and T. Ziman, PRB 39, 2608 (1989)
[6] P. Hasenfratz and F. Niedermayer, Z. Phys. B 92, 91-112 (1993)

Subject: Measurement of the phistar distribution of Z bosons decaying to electron pairs with the CMS experiment at a center-of-mass energy of 8 TeV

Measurements of the Z boson transverse momentum (QT) spectrum serves as
both a precision test of non-perturbative QCD and helps to reduce the
uncertainty in the measurement of the W boson mass. However, QT is
limited at its lowest values by detector resolution, and so a new variable,
Phi*, which performs better in the low QT region, is used instead.
This thesis presents the first measurement the normalized differential cross
section of Z bosons decaying to electron pairs in terms of Phi* at
sqrt(s) = 8 TeV. The data used in this measurement were collected by the CMS
detector at the LHC in 2012 and totaled 19.7/fb of integrated
luminosity. The results are compared to predictions from simulation, which are
found to provide a poor description of the data.

A primary goal of introductory physics courses is to help students develop problem-solving and related critical thinking skills. A physics education research group at the University of Minnesota (UMN) has developed internet-based coaches to help students learn problem-solving, guiding students through a systematic framework for a number of individual physics problems. Initial tests with students from introductory physics courses at UMN indicate that users found the coaches helpful and that those students who actively used the coaches improved their performance on problem-solving components of course exams as compared to similar students who had not used the coaches. In this project, we investigate whether these positive results can be replicated at Central Michigan University. Students in an introductory physics course were given homework assignments that included the coach problems. Students could voluntarily use the coaches to help them complete their assignments. Keystroke data were recorded to monitor how/whether students used the coaches. Students also completed surveys containing questions regarding their opinions of the coaches. The data have been analyzed to address the following issues: the usage and usability of the coaches, their usefulness as perceived by students, and the characteristics of the students who do and don’t use the coaches. The ultimate goal of this research is to develop effective practices for teaching and learning problem-solving in introductory physics courses.

Owing to their unique energy band structure and the ease of material synthesis, two dimensional nanomaterials, such as graphene, have become the ideal platform for observing novel electron transport phenomena. In particular, low energy quasiparticles in monolayer graphene behave like massless Dirac fermions, which have led to observations of many interesting phenomena, including Klein tunneling, anomalous Quantum Hall effect, etc. In contrast to the monolayer graphene, quasiparticles in bilayer graphene (BLG) are massive chiral fermions due to its parabolic band structure. Thus, BLG also gives a number of intriguing properties which are very different from those of monolayer graphene, including tunable band gap opening and anti-Klein tunneling, arising from chiral characteristics of charge carriers. However, unlike SLG, experimental works on chiral electron transport in BLG have received less attention.

In addition, other two-dimensional atomic layer crystals, such as atomically thin layered transition metal dichalcogenides (TMDCs), are also attractive material platform with unique electronic and optical properties, including indirect to direct band gap transition, and valley polarized carrier transport. However, study of the low temperature electron transport in atomic thin layered TMDCs is still in its infancy. One of the major hurdles for electron transport study lies in the large metal/semiconductor junction barrier for carrier injection, which leads to the contact resistance dominated charge transport in short channel nanoscale devices.

In this talk, I will show our first demonstrated successful synthesis of wafer scale BLG with high-homogeneity by low-pressure chemical vapor deposition (CVD). Next, I’ll discuss about the importance of chiral electron transport in BLG. I’ll present the signature of electronic cloaking effect with anti-Klein effect as a manifestation of chirality by probing phase coherent transport behavior in CVD bilayer graphene nanostructure. Finally, I’ll talk on the electron transport in two-dimensional TMDCs. I successfully fabricated monolayer MoS2 single electron transistors using low work function metal for the contact electrodes, and observed Coulomb blockade phenomena attributed to single electron charging on a fairly clean quantum dot.

Subject: The new Beam Halo Monitor system for the CMS experiment at the LHC

A new Beam Halo Monitor (BHM) detector system has been installed in the CMS cavern to measure the machine-induced background (MIB) from the LHC. This background originates from interactions of the LHC beam halo with the final set of collimators before the CMS experiment and from beam gas interactions. The BHM detector uses the directional nature of Cherenkov radiation and event timing to select particles coming from the direction of the beam and to suppress those originating from the interaction point. It consists of 40 quartz rods, placed on each side of the CMS detector, coupled to UV sensitive PMTs. For each bunch crossing, the PMT signal is digitized by a charge integrating ASIC and the arrival time of the signal is recorded. The data are processed in real time to yield a precise measurement of per-bunch-crossing background rate for each beam. This measurement is made available to CMS and the LHC, in order to provide real-time feedback on the beam quality and improve the efficiency of data taking. The BHM detector is now in the commissioning phase. An overview of the system and first results from Run II will be presented.

NOvA, the world’s leading long-baseline neutrino-oscillation experiment, uses a beam originating at the Fermi National Accelerator Laboratory near Chicago to measure electron-neutrino appearance and muon-neutrino disappearance with a 14,000-ton detector in northern Minnesota. Understanding neutrinos is one of the most important goals of elementary particle physics and could lead to deeper understanding of the origin and evolution of the universe. University of Minnesota faculty, staff and students have played leading roles in the design and construction of the NOvA laboratory and detector, as well as in the analysis of data collected during the first year and a half of operation. This special seminar will present first results from NOvA’s electron- and muon-neutrino measurements, along with an overall introduction to the NOvA project and a discussion of future running plans and ultimate scientific reach.

NOvA is a long-baseline neutrino oscillation experiment optimized for electron neutrino appearance in the NuMI beam, a muon neutrino source at Fermilab. It consists of two functionally identical, nearly fully-active liquid-scintillator tracking calorimeters. The near detector (ND) at Fermilab is used to study the neutrino beam spectrum and composition before oscillation, and measure background rate to the electron neutrino appearance search. The far detector, 810 km away in Northern Minnesota, observes the oscillated beam and is used to extract oscillation parameters from the data. NOvA's long baseline, combined with the ability of the NuMI beam to operate in the anti-neutrino mode, makes NOvA sensitive to the last unmeasured parameters in neutrino oscillations- mass hierarchy, CP violation and the octant of mixing angle theta23. This thesis presents the search for electron neutrino appearance in the first data collected by the NOvA detectors from February 2014 till May 2015. Studies of the NuMI neutrino data collected in the NOvA near detector and its effect on the oscillation measurement in the far detector are also presented.

Subject: A Model for Gas Dynamics and Chemical Evolution of the Fornax Dwarf Spheroidal Galaxy

This is the public portion of Zhen Yuan's thesis defense.

We present an empirical model for the halo evolution, global gas dynamics and chemical evolution of Fornax, the brightest Milky Way (MW) dwarf spheroidal galaxy (dSph). Assuming a global star formation rate, we derive the evolution of the total mass and the rate of net gas flow for cold gas in a growing star-forming disk. We identify the onset of the transition from net inflow to a net outflow as the time at which the Fornax halo became an MW satellite and estimate the evolution of its total mass using the median halo growth history in the ΛCDM cosmology and its present mass within the half-light radius derived from observations. We find that the Fornax halo grew to M h (t_sat ) ≈ 1.8 × 10 9 M ⊙ at t sat ≈ 4.8 Gyr and that its subsequent global gas dynamics was dominated by ram-pressure stripping and tidal interaction with the MW. Then we build a chemical evolution model on a 2-D mass grid, using supernovae as the enrichment source of the gaseous system. We find that the key parameter of controlling the element abundances pattern is the supernovae mixing mass. It is set differently between two types of supernovae and between two phases, before and after t sat in our model. The choice is determined based on the supernovae remnant evolution as well as the global gas dynamics. We also find the metal loss in the outflow dominated phase is severe, which is empirically implemented in our model. The data generated from the standard case can explain the observational data very well, e.g., abundance ratio of α element to Fe as a function of metallicity [α/Fe] vs. [Fe/H], metallicity evolution as a function of time [E/H] vs. t and metallicity distribution function (MDF) for Mg, Ca and Fe.

The two-dimensional electron system (2DES) hosted at the interface of MgZnO/ZnO now displays electron mobilities exceeding 1,000,000 cm2/Vs and electron scattering times comparable to the best AlGaAs/GaAs 2DES. In this talk I will discuss the growth technology used to create such high quality devices and introduce the physical phenomena they host at low temperatures. These themes include interaction-induced renormalization of the spin susceptibility of carriers and how it reveals the new facets of the fractional quantum Hall effect, along with non-equilibrium phenomena, such as microwave-induced resistance oscillations.

We consider a Dark Matter scenario in a singlet extension model of the Minimal Supersymmetric
Standard Model, which is known as nMSSM. We find that with high-scale supersymmetry breaking,
the singlino can obtain a sizable radiative correction to the mass. This opens a
Dark Matter scenario with resonant annihilation via the exchange of the Higgs boson.
We show that the current Dark Matter abundance and the Higgs boson mass can be
explained simultaneously. This scenario can be probed by XENON1T.
We also mention the possibility of Electroweak Baryogenesis at high temperature in this model.
If there exist vector like matters coupled to the singlet multiplet, the thermal effects deform
the Higgs potential at high temperature which derives a first order phase transition.
We show that a strong first order phase transition can occur when the temperature is around
the supersymmetry breaking scale, which can be TeV scale.

I will talk about the search for a unified theory of the laws of physics including quantum mechanics, which governs the very small, and general relativity, which governs the very large. Stephen Hawking showed 40 years ago that these theories make conflicting predictions near black holes. This ignited a battle that continues to this day: either quantum mechanics must break down, or our understanding of spacetime must be wrong. The latest wrinkle is the `firewall’ paradox: if quantum mechanics is to be saved, then an astronaut falling into a black hole will have an experience very different from what Einstein’s theory predicts. This has led to many new ideas that may lead to the unification of these two great theories.

Statistical mechanics depends on chaos --- the sensitive dependence on initial conditions --- to produce ergodic mixing. It has been known for more than four decades that black holes satisfy thermodynamic laws, but the associated chaotic behavior has been discussed only recently. I review these developments, and extend them from eternal black holes to black holes that form and decay.

Subject: Two observational studies using Van Allen Probes data: A case study of an unusually strong, widespread EMIC wave event and its impact on the radiation belts, and a statistical study of low-harmonic m

In the journal club we will discuss the behavior of a N-component φ4
-model in hyperbolic space based on Ref.1 I will motivate why such a study might be relevant for condensed matter physics and i will briefly sketch the derivation of the critical behavior of the model. I will follow the aforementioned reference and supplement the discussion for clarity.

The Luxton laboratory is focused on understanding the molecular mechanisms underlying the pathogenesis of dystonia. Dystonia is a neurological movement disorder characterized by repetitive muscle contractions that result in involuntary twisting of the extremities and abnormal posturing. People afflicted with dystonia can experience severe disruptions in their ability to perform routine tasks including walking and sitting. Dystonia is the third most common human movement disorder behind essential tremor and Parkinson’s disease. Despite its prevalence, we understand little about dystonia pathogenesis. The most common and severe form of inherited dystonia is early-onset or DYT1 dystonia. The symptoms of DYT1 dystonia first appear at a mean age of 12.5. DYT1 dystonia is caused by a mutation within the DYT1/Tor1a gene that encodes the evolutionarily conserved torsinA protein resulting in the deletion of a single glutamic acid residue (ΔE302/303, or ΔE). The mechanism through which the ΔE mutation causes DYT1 dystonia is unclear because the basic cellular function of torsinA is unknown. Our research has established torsinA as key regulator of nuclear-cytoskeletal coupling. We study the molecular mechanism of torsinA-dependent nuclear-cytoskeletal coupling using three powerful model systems: wounded fibroblast monolayers, the social amoeba Dictyostelium discoideum, and the Caenorhabditis elegans germline. Our research is holistic as we use biochemical, biophysical, cell biological, molecular genetic, and quantitative imaging approaches. Finally, we are actively screening for small molecules that modulate torsinA function in order to develop novel treatments for DYT1 dystonia.

Strontium titanate (SrTiO3) is the first and best known superconducting semiconductor. It exhibits an extremely low carrier density threshold for superconductivity, and possesses a phase diagram similar to that of high-temperature superconductors—two factors that suggest an unconventional pairing mechanism. Despite sustained interest for 50 years, direct experimental insight into the nature of electron pairing in SrTiO3 has remained elusive. Here we perform transport experiments with nanowire-based single-electron transistors at the interface between SrTiO3 and a thin layer of lanthanum aluminate, LaAlO3. Electrostatic gating reveals a series of two-electron conductance resonances—paired electron states—that bifurcate above a critical pairing field Bp of about 1–4 tesla, an order of magnitude larger than the superconducting critical magnetic field. For magnetic fields below Bp, these resonances are insensitive to the applied magnetic field; for fields in excess of Bp, the resonances exhibit a linear Zeeman-like energy splitting. Electron pairing is stable at temperatures as high as 900 millikelvin, well above the superconducting transition temperature (about 300 millikelvin). These experiments demonstrate the existence of a robust electronic phase in which electrons pair without forming a superconducting state. Key experimental signatures are captured by a model involving an attractive Hubbard interaction that describes real-space electron pairing as a precursor to superconductivity.

CANCELLED due to travel issues but a discussion is scheduled with Professor Singh from 5-6:00 in 230 Bruininks Hall for those who would like to pursue the topic less formally. Learning physics is challenging. There are only a few fundamental principles of physics that are condensed in compact mathematical forms. Learning physics requires unpacking these fundamental principles and understanding their applicability in a variety of contexts. In this talk, I will discuss our research that has implications for helping students learn to think like a physicist and improving their problem solving, reasoning and metacognitive skills.

The ALFALFA HI survey has detected rare but extremely interesting HI sources that probe the extreme limits of systems that are able to form stars. These sources include the ultra-compact high velocity clouds (UCHVCs), some of which may represent gas-rich but (nearly) starless Local Group galaxies; the low-mass gas-rich SHIELD dwarf galaxies; and the "Almost-Dark" HI sources, which are clearly extragalactic HI detections that have no discernible optical counterpart in extant optical surveys. I will discuss these different source populations and how they are interconnected, highlighting one system, AGC 226067, that bridges the three categories. I will also focus in particular on the UCHVCs as candidate Local Group galaxies, highlighting one particular source, AGC 198606, for which HI observations with WSRT and deep WIYN/ODI optical imaging strongly support the hypothesis that it represents gas in a nearby dark matter halo.

Learning quantum mechanics can be challenging, in part due to the non-intuitive nature of the subject matter. I will describe investigations of the difficulties that students have in learning quantum mechanics. We find that the patterns of reasoning difficulties in learning quantum mechanics are often universal, similar to the universal nature of reasoning difficulties found in introductory physics. Moreover, students often fail to monitor their learning while learning quantum mechanics. To help improve student understanding of quantum concepts, we are developing quantum interactive learning tutorials (QuILTs) as well as tools for peer-instruction. The goal of QuILTs and peer-instruction tools is to actively engage students in the learning process and to help them build links between the formalism and the conceptual aspects of quantum physics without compromising the technical content.

Many movies over the last several decades have dramatized our fascination with alien life forms. But could aliens really exist? This lecture will highlight how astronomers detect and characterize planets outside our solar system that could harbor alien life. The recent revolution of exoplanet detections by the NASA Kepler mission and ground-based searches for exoplanets has given way to a new understanding of how common place other worlds are in the Galaxy. Our prospective of astrobiology as suddenly blossomed. Highlighted, as part of this presentation, will be a review of how astronomers detected and characterize these exo-planets, using techniques at the Large Binocular Telescope Observatory and elsewhere, a reflection on the potential requirements of the habitability zones in exo-planetary system, highlights from NASA missions designed to search for alien worlds, and the surprises within our own solar systems of bodies that may harbor life at present of may have supported life in the past. Indeed, we may be at the point where "E.T. will phone home."

Subject: Potential Signatures of High-Energy Neutrinos Produced by Relativistic Jets in Gamma-Ray Bursts and Core-Collapse Supernovae

We point out that high-energy neutrinos produced by relativistic jets can be annihilated with thermal neutrinos emitted by the associated accretion disks in gamma-ray bursts and core-collapse supernovae. For a broad range of conditions, the emerging all-flavor spectrum for neutrinos of E >∼ 0.1 PeV is modified by a factor E^(−n) with n ≈ 0.4–0.5. Flavor evolution of accretion-disk neutrinos does not affect this result but can change the emerging flavor composition of high-energy neutrinos. The above signatures provide potential tests of detailed models by IceCube.

Descartes’ Principles of Philosophy (1644) and Newton’s Principia mathematica (1687) are two of the most important works of seventeenth century natural philosophy. Yet, when put side by side, it is far easier to identify differences between the texts than it is to pin-point similarities. Their laws of nature are a case in point. Descartes deduces his three laws from our knowledge of God and claims these laws are true insofar as they capture the world as God actually created it. Newton, in contrast, “deduces” his laws of motion “from the phenomena,” which suggests that these laws are true of the world as it is presented to our senses. In this paper, I first clarify the epistemic significance of Descartes’s and Newton’s competing “deductions” and competing notions of truth. Based on that treatment, I then highlight a significant and frequently overlooked point of agreement: Both Descartes and Newton adopt methods for establishing true laws of nature that allow us to know that bodies obey particular laws without a complete understanding of why they do, i.e., without requiring that we identify the natural processes and properties that explain the behaviors that the laws describe.

There is a strong experimental effort to discover the signal from primordial gravity waves. For the case that the inflation process happened in the simplest (standard) way, this signal reveals the energy scale of inflation, which is one of the most crucial questions about this era. However, there are other natural options that can be distinguished with distinct observational properties. This talk will concentrate on two models; each of them violates the standard case in a different way.

I will talk about a simple, paradigmatic example of how to understand exotic spin liquids (aka non-Abelian topological phases) in terms of more simpler ones as presented in Ref.[1]. The toric code model [2] will be used as a spin-1/2 lattice model giving rise to a (relatively) simple spin liquid. Using this as a starting point, a spin-1 model is constructed from the spin-1/2 model. Borrowing also explanations from Ref.[3], we shall see that the more complex spin liquid harbors topological excitations, which correspond to those of the (doubled) Ising theory.

Crystallization is one of the most familiar phase transitions; despite that, its theoretical understanding remains a major challenge. Why some crystal structures are more common than others? What selects between different close-packed orderings? Why do atoms sometimes prefer to arrange themselves quasi-periodically? While there is probably not a unique answer to these questions, in metallic alloys, I will argue that the driving physics may not be very different from that of various density (spin, charge, etc) instabilities within crystalline states.

According to the original quark model template there are mesons consisting of a quark and an antiquark, and baryons made from three quarks. Strongly interacting particles that do not fit this template are called exotic. Experimental findings of recent years have uncovered existence of exotic mesons: tetraquarks, and baryons: pentaquarks, containing an additional heavy quark-antiquark pair. I discuss properties of such particles and the current theoretical approaches to understanding their internal dynamics.

What is the correct description of the effective theory for long wavelength modes in inflation? I'll argue that the standard EFT tools do not apply here and that a new description based on open effective theories is required. I'll discuss how this is related to the Stochastic Inflation program and how decoherence takes place in inflation.

A key outstanding question in our understanding of star formation is whether magnetic fields provide support against the gravitational collapse of their parent molecular clouds and cores. While direct measurements of magnetic field strength are challenging, observations of polarized thermal emission from dust grains aligned with respect to the local magnetic field can be used to map out the cloud magnetic field. In this talk I present early results from a BLASTPol, a sensitive balloon-borne polarimeter. BLASTPol observed the Vela C cloud during a 2012 Antarctic flight, yielding the most detailed submillimeter polarization map ever made of a molecular cloud forming high mass stars. Statistical comparisons between submillimeter polarization maps and 3-D numerical simulations of magnetized star-forming clouds are a promising method for constraining magnetic field strength, but uncertainty concerning how the dust polarizing efficiency varies as a function of density and other cloud parameters can make such comparisons difficult. Previous work suggests that this uncertainty can be reduced by studying the dependence of observed polarization fraction (p) on column density (N). In Vela C, I find that most of the structure in p can be modeled by a power-law dependence on two quantities: The first is N and the second is the local dispersion in polarization angle (S). This empirical model for p(N,S) provides new constraints for models of magnetized star-forming clouds and an important first step in the interpretation of the BLASTPol 2012 data set. Finally, I discuss a “next-generation” BLAST polarimeter, which is scheduled for a first Antarctic flight in late 2016. BLAST-TNG will have an order of magnitude increase in both spatial resolution and mapping speed and will map dozens of star-forming regions, placing important constraints on the role magnetic fields play in regulating star formation.

Refreshments served at 3:15 p.m. A Charles E. Culpeper Lecture in the History of Medicine. Co-sponsored with the Center for Early Modern History.

Descartes is often said to be the French philosopher who gave us the mind-body problem. But he only began to write philosophy seriously in his 30s, living abroad. In his youth he apparently became we acquainted with libertine writers; when the assassination of the Queen Regent’s favorite, Concini, took place in 1617 he left to learn the art of war and became deeply immersed in French entanglements related to the Thirty Years War. After another short period in Paris his personal and political involvements seem to have caused him and his friends to feel threatened by the chief minister, Cardinal Richelieu. He spent the last twenty years of his life (1629-49) as an exile in The Netherlands, where he indeed had the leisure and ambition for writing about the nature of the world. Can we re-connect his mind and body?

To help cover costs of this event, we are asking attendees to contribute 10forfaculty,and
5 for postdocs, staff, and students (no charge for family members). If you would rather bring food, you may bring a side dish or dessert to share instead of paying. You can bring your payment to the picnic if you haven't already paid.

Subject: Potential Signatures of High-Energy Neutrinos Produced by Relativistic Jets in Gamma-Ray Bursts and Core-Collapse Supernove

We point out that high-energy neutrinos produced by relativistic jets can be annihilated with thermal neutrinos emitted by the associated accretion disks in gamma-ray bursts and core-collapse supernovae. For a broad range of conditions, the emerging all-flavor spectrum for neutrinos of E >∼ 0.1 PeV is modified by a factor E^(−n) with n ≈ 0.4–0.5. Flavor evolution of accretion-disk neutrinos does not affect this result but can change the emerging flavor composition of high-energy neutrinos. The above signatures provide potential tests of detailed models by IceCube.

Please Note: Meeting room will be changed to PAN 210 for the rest of the semester.

Recently the experimental discovery of surface Fermi arcs in TaAs were the first proof of existence of a Weyl semimetal [1]. The existence of these states in 3 spatial dimensions was theoretically proposed before, a short summary was published in a viewpoint [2]. The characteristic surface Fermi arcs appear as a projection of Weyl points in the bulk material. A Weyl point can only appear when either time reversal or spacial inversion symmetry are broken, and when there is an accidental degeneracy of two bands [3,4]. Following [4] I will show why this degeneracy is unlikely in 2 dimensions but rather generic in 3 dimensions, and how the projection of the Weyl points onto the surface yields a structure of surface Fermi arcs. However not every surface of a Weyl semimetal contains surface arcs, if two Weyl points of opposite chirality have the same projection the surface states cancel out. As example I will discuss the structure of the 24 Weyl points in the first experimentally known Weyl semimetal, TaAs.

Massive, long-lived particles do not exist in the SM, and so any sign of them would be an 3 indication of new physics. There are many BSM theories that predict long-lived particles, including split SUSY, hidden valley scenarios, GUT theories, and various SUSY models. Long-lived particles could be sufficiently massive that they would loose sufficient energy through ionization or hadronization, depending on the type of particle, that they would come to rest inside the detector material. A search for long-lived particles that stop in the CMS detector and decay to muons was performed. The decays of the stopped particles could be observed when there are no pp collisions in the detector, namely during gaps between bunch crossings. The analysis uses 19.7 1/fb of 8 TeV data collected by CMS in 2012, during a search interval of 293 hours of trigger livetime. We also set cross section limits for each mchamp mass as a function of lifetime, for lifetimes between 100 ns and 10 days. These are the first limits for stopped particles that decay to muons, and they are also the first limits for lepton-like multiply charged particles that stop in the detector.

Subject: New Approaches to Quantum Scattering and the Payoff for the LHC

Refreshments to be served outside 230 SSTS after the colloquium.

The Large Hadron Collider is renewing its exploration of the energy frontier of particle physics, searching for new particles and interactions beyond the Higgs boson. For the LHC to uncover many types of new physics, the "old physics" produced by the Standard Model must be understood very well. For decades the central theoretical tool for this job was the Feynman diagram. However, Feynman diagrams are just too slow, even on fast computers, to allow adequate precision for complicated events with many jets of hadrons in the final state. Such events constitute formidable backgrounds at the LHC to many searches for new physics. Over the past few years, alternative methods to
Feynman diagrams have come to fruition. The new "on-shell" methods are based on the old principle of unitarity. They can be much more efficient because they exploit the underlying simplicity of scattering amplitudes, and recycle lower-loop information. Farther afield, the
new methods have led to intriguing new results in quantum gravity and in supersymmetric analogs of the Standard Model. I'll explain how and why these methods work, and present recent state-of-the-art results obtained with them.

Cosmological simulations of galaxy formation are rapidly advancing towards smaller scales. Current models can now resolve giant molecular clouds in galaxies and predict basic properties of star clusters forming within them. I will describe new theoretical simulations of the formation of the Milky Way throughout cosmic time. However, many challenges - physical and numerical - still remain. I will discuss how observations of massive star clusters and star forming regions can help us overcome some of them.

Subject: Stellar Origin of $^{10}$Be and a Low-Mass Supernova Trigger for the Formation of the Solar System

The abundances of short-lived radio-isotopes (SLR) in the early solar system (ESS) measured in meteorites provide crucial information about the events leading to the formation of the solar system. However, the origin of several of these SLR in the ESS is still uncertain. We show that one of the key SLR ^{10}
Be can be made by neutrino-induced spallation during a core-collapse supernova (CCSN) in contrast to the current paradigm of ^{10}
Be production exclusively by cosmic rays. In addition to ^{10}
Be, we find that a recent low-mass CCSN, that occurred ~ 1 Myr before the Solar system formation, can self-consitently account for the ESS abundance of other SLRs such as ^{41}
Ca, ^{107}
Pd, ^{53}
Mn, ^{60}
Fe while also producing significant amounts of ^{26}
Al, ^{36}
Cl, ^{182}
Hf, ^{135}
Cs, and ^{205}
Pb. This makes such a low-mass CCSN an attractive candidate for the event that triggered the formation of the solar system.

Subject: Optimal number of terms in QED series and its consequence in condensed matter implementations of QED

In 1952 Dyson put forward a simple and powerful argument indicating that the perturbative expansions of QED are asymptotic. His argument can be related to Chandrasekhar's limit on the mass of a star for stability against gravitational collapse. Combining these two arguments we estimate the optimal number of terms of the QED series to be
. For condensed matter manifestations of QED in narrow band-gap semiconductors and Weyl semimetals the optimal number of terms is around 80
while in graphene the utility of the perturbation theory is severely limited.

I present a simple extension of the Standard Model that gives rise to post-sphaleron baryogenesis by introducing colored scalar fields. The model can accommodate a fermionic dark matter (DM) candidate of O(GeV) mass whose stability is tied to proton stability. The supersymmetric extension of this model is straightforward and can realize a multicomponent DM scenario. I discuss prospects for direct and indirect detection of the DM candidate(s) and possible collider signals of the model.

Living matter, at the molecular scale, is different from usual matter. Biological molecules, specifically enzymes, deform without breaking, couple chemical reactions to molecular tasks. Nature’s molecular machines are not scaled down versions of macroscopic machines: molecular motors have nothing to do with Carnot cycles, and molecular pumps have nothing to do with hydrodynamics. So how do these molecules work. I will describe our advances in extracting universal mechanical properties of enzymes, and come to the surprising conclusion that the molecules of life are visco-elastic ! Enzymes “flow” from one solid – like conformation to another. As it turns out, the molecules we are made of behave dynamically like “silly putty”. Another universal conclusion is that any enzyme can be controlled mechanically, opening mechanical control for thousands of chemical reactions.

I will discuss recent work studying universal properties of gravity in anti-de Sitter (AdS) spacetime from the perspective of conformal field theory (CFT). Using the AdS/CFT correspondence, scattering with black holes can be rephrased in terms of correlation functions involving operators with large scaling dimension. I will present new methods for calculating these correlation functions in 2d CFTs which make the connection to black holes and gravity manifest. In particular, I will show that operators with large scaling dimension create a thermal background with the correct Hawking temperature, with interactions that can be described using new on-shell diagrams. I will then discuss the connection with eigenstate thermalization and the possibility of generalizing these results to theories in higher dimensions.

Social scientists have been pursuing causal knowledge from observational studies for well over 100 years, with limited success. In the last 25 years, however, the computational and statistical methods available for causal modeling and discovery have exploded. I describe this revolution and illustrate it on some recent case studies in social and biomedical science. I also describe the challenges that still remain, including conceptual problems of defining variables and inferential problems arising from trying to measure them.

Subject: Using Lyman-alpha to detect galaxies that leak Lyman continuum

Determining the population of objects that reionized the Universe and maintained the subsequent thermal history of the intergalactic medium remains an urgent observational question. We use the Lyman-alpha line to identify the best candidates for Lyman continuum leakage. We will present Lya radiative transfer models, and their application to observational data.

Following Anderson's 1973 conjecture of a resonating valence-bond (RVB) state, theorists have been actively exploring quantum spin liquids – states of magnets without long-range order. In the last 15 years we have progressed from knowing what quantum spin liquids are not (states with local order) to understanding what they are. Several solvable models of spin liquids have been shown to be instances of lattice gauge theories (Kitaev). An alternative perspective is a picture of fluctuating closed strings (Wen). An open string contains two "fundamental" particles on its ends, which can be bosons, fermions, or anyons, depending on the model. Although these models, featuring multi-spin interactions, look contrived, more realistic models (e.g., Kitaev's honeycomb) share many of their exotic features. I will show how certain lattice defects of Kitaev's honeycomb model can bind Majorana zero modes.

In the quark model, hadrons are dominantly bound states of quark-antiquark pairs (mesons) or three quarks (baryons), but QCD also allows for hadronic states composed of more quarks bound together. Recently, BESIII, Belle and LHCb have confirmed the existence of four-quark and pentaquark candidates. These new states, along with experimentally observed resonances that do not fit well into the charmonium and bottomonium spectra, present challenges and opportunities for strong interaction theory. In this talk, I will review results on these exotic quark states that have been obtained by the BESIII experiment at the Beijing Electron Positron Collider II (BEPCII).

Magnets host a variety of solitons that are stable for topological reasons: domain walls, vortices, and skyrmions, to name a few. Because of their stability, topological solitons can potentially be used for storing and processing information. This motivates us to build economic, yet realistic models of soliton dynamics in magnets. E.g., a domain wall in a cylindrical ferromagnetic wire can be pictured as a bead on a string, which can move along the string and rotate about its axis. Its mechanics is counterintuitive: it rotates if pushed and moves if twisted. I will review basic models of ferro- and antiferromagnetic domain walls in one dimension and discuss examples from higher dimensions, e.g., vortices and skyrmions.

Theories of neutral naturalness can realize untuned solutions to the hierarchy problem while avoiding LHC current exclusion bounds on colored top partners. Their unusual signatures demonstrate the range of phenomena that can connected to stabilizing the electroweak scale, motivating searches for displaced vertices, exotic Higgs decays and emerging jets. I will give an overview of how to constrain these theories experimentally, and present recent work toward deriving a phenomenological model-independent 'no-lose theorem' for perturbative solutions to the hierarchy problem which rely on top partners, including neutral naturalness. This lends strong model-independent motivation to build both proposed future lepton and 100 TeV colliders, since both are needed as discovery machines of general naturalness.

Subject: Nanofabrication for Astronomy: Small Features for Small Wavelengths

In the coming decades, the field of X-ray astronomy desires to accomplish several key science goals. Current X-ray observatories are incapable of addressing many of these including detailing the distribution of hot matter in the Universe. A large fraction of baryonic matter is theorized to exist in between galaxies. Detection of this matter and characterizing feedback mechanisms from galaxies are substantial advancements of our understanding that can be realized through soft X-ray spectroscopy. Future X-ray missions require diffraction gratings with high throughput and high spectral resolving power to achieve these goals. Recent advances in grating fabrication methods have enabled reflection gratings to obtain the necessary performance requirements. I will discuss these novel fabrication methods and detail our progress in fabricating custom grating groove profiles. These gratings have demonstrated very high X-ray diffraction efficiency and spectral resolving power during X-ray testing. I will detail these results and describe current and future space based applications of spectrometers based on such gratings.

Speaker: Paul Jardine, Dept. of Diagnostics and Biological Sciences, University of Minnesota

Subject: Role Reversal: Using Biology to Answer Questions in Physics

The broad field of molecular biology originated with the movement of physics into biological systems. With tremendous success, physicists were able to address fundamental questions in biological systems by applying physical methods of analysis. We are attempting to reverse these roles in that we are using a biological system to address complex questions in physics. We study the DNA packaging process in dsDNA viruses. During their assembly, these viruses compact DNA inside a protein shell, or capsid, to near crystalline density. Studying this system allows us to study phenomena not accessible to other experimental systems, including the confinement of charged polymers on the nanometer scale in real time. This work has revealed interesting principles of polymer dynamics, among them the forces that resist DNA confinement, the timescale of relaxation events, and the physical behaviour of molecules as they jam during translocation. I will present a brief overview of this work.

Fluorescence fluctuation spectroscopy (FFS) is a powerful method for quantifying protein interactions. By exploiting the brightness of fluorescence intensity fluctuations we are able to measure the stoichiometry of protein complexes. FFS is particularly valuable because it allows real-time measurements within living cells, where protein complex formation plays a crucial role in the regulation of cellular processes. However, intensity fluctuations are frequently altered by the cell environment in subtle and unanticipated ways, which can lead to failure of the available FFS analysis methods. This thesis demonstrates that measuring in very small volumes, such as yeast and E. coli cells, can introduce a significant bias into the measured brightness as a result of cumulative sample loss, or photodepletion. This loss leads to a non-stationary signal, which is incompatible with the implicit assumption of a stationary process in conventional FFS theory. We addressed this issue by introducing a new analysis approach that serves as a foundation for extending FFS to non-stationary signals.
FFS measurements in cells are also currently limited to the study of binary interactions involving two different proteins. However, most cellular processes are mediated by protein complexes consisting of more than two different proteins. Observation of pairwise interactions is not sufficient to unequivocally determine the binding interactions involving three or more proteins. To address this issue, we extended FFS beyond binary interactions by developing tricolor heterospecies partition analysis to characterize ternary protein systems. The method is based on brightness analysis of fluorescence fluctuations from three fluorescent proteins that serve as protein labels. We verified tricolor heterospecies partition analysis by experiments on well-characterized protein systems and introduced a graphical representation to visualize interactions in ternary protein systems.

Speaker: Naomi Courtemanche, University of Minnesota, College of Biological Sciences

Subject: Mechanistic studies of formin-mediated actin assembly

Actin dynamics drive many cellular processes, including motility, cytokinesis and endocytosis. A host of actin-binding proteins controls actin filament nucleation, elongation, capping, branching and bundling. Members of the formin family of proteins nucleate new filaments and remain processively attached to actin filament barbed ends while promoting their elongation. Because we lack many of the details required to understand how formins function in cells, it is not known how they promote healthy cellular proliferation. I will present the results of three studies aimed at elucidating the mechanism of actin polymerization mediated by the S. cerevisiae formin Bni1p. First, I will describe experiments I performed using total internal reflection fluorescence microscopy to address the role of sequence organization in Formin Homology (FH) 1 domain-mediated transfer of profilin-actin to a formin-bound filament end. Second, I will describe a technique I developed called “actin curtains”, which I used to study the effect of linear force on formin-mediated polymerization. I will show that small linear forces dramatically slow formin-mediated polymerization in the absence of profilin, suggesting that force shifts the conformational equilibrium of the end of a formin-bound filament, but that profilin-actin associated with FH1 domains reverses this effect. Third, I will describe the effects of point-substitutions in the FH2 domain on the activity of Bni1p, which suggest that nucleation and elongation are separable functions of formins that involve different interactions with actin. These studies provide insight into the physical properties of formins, which will be useful in guiding future studies aimed at elucidating how sequence variations confer unique biological functions to formin isoforms expressed in the same cell.

Unconventional superconductivity is found in close proximity to a putative magnetic quantum critical point in several materials, such as heavy fermions, organics, cuprates, and iron pnictides. This led to the proposal that, in contrast to conventional electron-phonon superconductors, critical magnetic fluctuations promote the binding of the electrons in Cooper pairs. Experiments in many of these materials revealed that their superconducting transition temperature Tc is remarkably robust against disorder, particularly when compared to the predictions from the conventional Abrikosov-Gor'kov theory of dirty superconductors. In this talk, we investigate the impact of weak impurity scattering on the onset of the pairing state mediated by quantum critical magnetic fluctuations.We find that both the build-up of incoherent electronic spectral weight near the magnetic quantum phase transition, as well as the changes in the pairing interaction caused by disorder, lead to a significant reduction in the suppression rate of Tc with disorder, as compared to the Abrikosov-Gor'kov theory. Both effects are unique to the problem of electronically-mediated pairing, shedding new light on the understanding of unconventional superconductivity in correlated materials, where disorder is always present.

The Local Group and the tiny galaxies that surround the Milky Way provide unique and detailed data sets for testing ideas in cosmology and galaxy formation. In this talk I will discuss how numerical simulations coupled with local "near-field" observations are informing our understanding of dark matter, the formation of the first galaxies, and the physical processes that act at the threshold of galaxy formation.

Subject: Early Time Dynamics of Gluon Fields in High Energy Nuclear Collisions

Nuclei colliding at very high energy create a strong, quasi-classical gluon field during the initial phase of their interaction. We present an analytic calculation of the initial space-time evolution of this field in the limit of very high energies using a formal recursive solution of the Yang-Mills equations. We provide analytic expressions for the initial chromo-electric and chromo-magnetic fields and for their energy-momentum tensor. In particular, we discuss event-averaged results for energy density and energy flow as well as for longitudinal and transverse pressure of this system. Our results are generally applicable if the time is less than the inverse of the saturation scale Q_s. The transverse energy flow of the gluon field exhibits hydrodynamic-like contributions that follow transverse gradients of the energy density. In addition, a rapidity-odd energy flow also emerges from the non-abelian analog of Gauss' Law and generates non-vanishing angular momentum of the field. We will discuss the space-time picture that emerges from our analysis and its implications for observables in heavy ion collisions.

Subject: Nucleosynthesis and Neutrinos Near Newly Formed Compact Objects

The origin of the rapid neutron capture process (r-process) elements remains the biggest unsolved question in our understanding of the origin of the elements in the Milky Way. The r-process is responsible for producing around half of the elements heavier than iron, but the astrophysical site in which it occurs is still uncertain. The most likely sites for the formation of these nuclei involve dynamical events in the lives of neutron stars: the inner most regions of massive stars during core collapse supernovae and the merger of a neutron star and another compact object. In both of these environments, neutrinos, nuclear physics, and gravity play paramount roles in determining the evolution of the dense object itself and in determining what nuclei are synthesized in material that is ejected from the system. I will first discuss prospects for the r-process in the inner most regions core-collapse supernovae. This part of the talk will focus on my work studying neutrino emission during core-collapse supernovae, which has constrained the possible modes of nucleosynthesis in these events. Second, I will discuss nucleosynthesis in material ejected during binary neutron star mergers and my work predicting optical transients powered by the decay of ejected radioactive nuclei. I will highlight some of the uncertainties that exist in both of these scenarios, and how these uncertainties can be reduced with future theoretical and computational work with input from current and next generation observational and experimental facilities.

Subject: Origins of a Legitimation Crisis: Medical Science, Private Profit, and the Challenge of Big Pharma

Refreshments served at 3:15 p.m.

Richard Horton, editor-in-chief of The Lancet, has recently suggested that as much as half of all published medical literature may be false. Horton is not alone in making such a claim: over the past decade a growing number of influential critics from within the medical establishment have raised significant concerns about the evidentiary basis of contemporary medicine. Drawing from my recent work on intellectual property rights and the history of the pharmaceutical industry, in this talk I present some preliminary thoughts on the origins of what I see as a brewing legitimation crisis facing medical science. I suggest that one possible origin point for current concerns about the evidentiary basis of scientific medicine can be found during the late nineteenth century, when a series of therapeutic reformers re-conceptualized the relationship between medical science and monopoly rights in drug manufacturing. In doing so, these reformers sought to legitimize the role of private profit in the production of scientific knowledge; unintentionally, they also cast in doubt the very possibility of an objective science free from motivated self-interest. Since then, I suggest, the tension between private profit as both a productive force and a source of skepticism has been generalized to such an extent that the very possibility of actionable scientific knowledge in the medical domain now seems threatened.

Since the building-blocks of supersymmetric models include chiral superfields containing pairs of effective scalar fields, a multifield approach is particularly appropriate for models of inflation based on supergravity. We discuss two-field effects in no-scale supergravity models, showing that no-scale models naturally yield Planck-friendly results, in the form of an effective Starobinsky potential for the inflaton, or through a reduction of r to very small values, r<<0.1, due to an enhancement of the scalar power spectrum, in a model with a quadratic potential. We finally discuss inflaton decays and reheating bounds on inflation, including scenarios where the inflaton possesses direct Yukawa couplings to MSSM fields, and where the inflaton decays via gravitational-strength interactions.

Speaker: Xiang Cheng, University of Minnesota, Department of Chemical Engineering and Material Science

Subject: Impact response of granular materials: From the origin of the universe to catastrophic asteroid strikes

Granular materials are large conglomerations of discrete macroscopic particles. Examples include seeds, sand, coals, powder of pharmacy, etc. Though simple, they show unique properties different from other familiar forms of matter. The unusual behaviors of granular materials are clearly illustrated in various impact processes, where the impact-induced fast deformation of granular materials leads to emergent flow patterns revealing distinctive granular physics. Here, we explored the impact response of granular materials in two specific experiments:

First, we performed the granular analog to “water bell” experiments. When a wide jet of granular material impacts on a fixed cylindrical target, it deforms into a sharply-defined sheet or cone with a shape mimicking a liquid of zero surface tension. The jets' particulate nature appears when the number of particles in the beam cross-section is decreased: the emerging structures broaden, gradually disintegrating into diffuse sprays. The experiment reveals a universal fluid structure arising from the collision of discrete particles, which has a counterpart in the behavior of quark-gluon plasmas created by colliding heavy ions at the Relativistic Heavy Ion Colliders.

Second, we investigated impact cratering in granular media induced by the strike of liquid drops—a ubiquitous phenomenon relevant to many important environmental, agricultural and industrial processes. Surprisingly, we found that granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters. Inspired by this similarity, we develop a simple model that quantitatively describes various features of liquid-drop imprints in granular media. Our study sheds light on the mechanisms governing raindrop impacts on granular surfaces and reveals an interesting analogy between familiar phenomena of raining and catastrophic asteroid strikes.

Jets are very important tool in studying physics at a hadron collider like the LHC. They are the direct manifestations of quarks and gluons and they constitute the most essential tool to test QCD. In this seminar I will talk about the reconstruction of jets in the LHC. It involves combining the information from several detectors. It also includes the best possible calibration of the detector elements, jet energy correction from information of collision data as well as Monte Carlo, correcting from the effect of pile up events etc. Reconstructed jets are then utilized in computing several variables used in understanding perturbative as well as non-perturbative QCD effects, and also as efficient tools in search of new physics. Some of the tests of the Standard Model will be discussed.

Our understanding of the formation and evolution of galaxies has been revolutionized in the past decade. Galaxies' growth is now thought to be regulated by the physics of baryons, through a self-regulating process wherein the star-formation rate, the gas accretion rate, and the gas outow rate all satisfy a slowly evolving equilibrium condition. However, there are still a number of problems/open questions with these baryon-dominated models, particularly when low-mass galaxies are looked at in detail. I will present recent results from the WFC3 Infrared Spectroscopic Parallel Survey (WISP) that we are conducting on the Hubble Space Telescope. This large program is identifying thousands of galaxies across a wide range of redshifts, spanning more that two thirds of the age of the universe. Our survey provides a selection function that is independent of galaxy stellar mass, and thus allows the study of those low mass objects that are mostly affected by energy feedback. These are the galaxies that provide the strongest constraints on galaxy formation models. I will also discuss our results in the context of future space based surveys such as Euclid and WFIRST-AFTA.

I construct an effective field theory (EFT) for the minimal degrees of freedom of supersymmetric inflation. These can be viewed as the goldstone of spontaneously broken time translations, and the goldstino of spontaneously broken SUSY, both of which are tied together in an interesting way through the structure of the SUSY algebra. I will outline some phenomenological consequences of the leading-order Lagrangian, including a modified goldstino/gravitino dispersion relation and a time-dependent gravitino mass phase. Finally, I will describe schematically the possible contributions of goldstino loops to inflationary correlators.

Our understanding of galaxy evolution centers around questions of how gas gets into galaxies, how it participates in star formation and black hole growth, and how it is returned to its galactic surroundings via feedback. On a global scale, measurements of the baryon density and the stellar mass function indicate that only 5% of baryons have formed stars by the present day, and this suggests that feedback from massive stars and supermassive black holes must prevent gas from forming stars in both low-mass and high-mass dark matter halos. I will present observational results on the geometry and kinematics of outflowing and inflowing gas around galaxies, including measurements of ejective feedback that is capable of quenching star formation by removing the cold gas supply. These results have broader implications for how gas is consumed and expelled at the centers of massive galaxies and for the limits of feedback from stellar radiation and supernovae. I will also discuss prospects for characterizing the physical properties of gas in and around galaxies using multi-wavelength spectroscopy with existing and future facilities.

Subject: Fictional Models and Models as Fictions: Disentangling the Difference

EVENT CANCELLED!!!

Because models often represent the world in unrealistic ways an increasingly popular view in the philosophical literature classifies models as fictions, aligning the way they convey information with strategies used in various forms of fictional writing. While fictional models certainly play a role in science I want to resist the “models as fictions” view by arguing that it not only has the undesirable consequence of erasing an important distinction between different types of models and modelling practices, but it fails to enhance our understanding of the role that fictional models do play in the transmission of scientific knowledge.

The Local Group affords us the opportunity to study the low-mass extremes of galaxy formation and cosmology. In this capacity, it presents some of the most enduring challenges to the very successful LCDM cosmology. I will discuss to what degree standard theoretical models of the local Universe match the growing volume and diversity of observations in the Local Group and beyond, with an emphasis on what these data may reveal about the nature of dark matter and the low-mass threshold of galaxy formation. These observations also have important implications for cosmic reionization, which is expected to play a central role in determining the abundance of low-mass galaxies around the Milky Way. I will argue that, even in the JWST era, the local Universe may be our best probe of low-mass galaxies at high redshift that are expected to be crucial for reionization.

This the public portion of Mr. Norbert's defense. His advisor is Yuichi Kubota.

The Standard Model (SM) despite its unmatched success in describing visible matter in the universe does not describe massive long-lived neutral particles. Therefore any sign of a massive long-lived neutral particle at the Large Hadron Collider(LHC) for example, would be an indication for new physics. Many models Beyond the SM like Gauge Mediated Supersymmetry Breaking Models(GMSB), Split SUSY models, Hidden Valley models and Large Extra Dimension models predict the existence of a massive long-lived neutral particle which decays into a photon and a gravitino.
Capitalizing on the excellent timing resolution of the Electromagnetic Calorimeter(ECAL) of the CMS detector, this thesis presents the search for events whose final state consist of at least one photon with late arrival time at ECAL compared to photons produced directly from proton-proton collisions at the LHC and large missing transverse energy in data recorded by the CMS detector from proton-proton collisions at 8 TeV center of mass energy in 2012. This data corresponds to 19.1/fb total integrated luminosity.
A photon from the decay of a massive long-lived neutral particle arrives late at ECAL due to the long lifetime of the massive long-lived neutral particle and is detectable using timing measurements of photons by ECAL. The large missing energy is used to infer the presence of an undetectable particle like the gravitino produced along with the photon from the decay of the massive long-lived neutral particle. The search covers particle lifetimes ranging from 3 to 40 nanoseconds(10^-9s).

It is well known that spontaneous broken continuous symmetry leads to the existence of a massless bosonic excitation, which is called Nambu-Goldstone bosons(NGB). Examples of NGB are pions in high energy physics, phonons and magnons(spin wave) in condensed matter physics. NGB usually interact 'weakly' with other degrees of freedom, in the sense that only its gradient couples to other particles, and so they can survive. This is the so-called Adler's principle(or Adler zero). As first discovered by Adler, forward scattering amplitude of particles off a pion vanishes quadratically as the momentum of pions goes to zero. Similarly in condensed matter physics, phonons(or magnons) have vanishing coupling with electrons as the wavevector of phonons(or magnons) goes to zero. However, Adler's principle is not always true. There is a criterion [1], saying that if the broken symmetry generator doesn't commute with translations, the coupling is non-vanishing and sometimes it results in breakdown of Fermi liquid and overdamping of NGB. The case of non-vanishing coupling is relevant to nematic fermi fluid [2].

Nonthermal plasma synthesis of nanocrystals is particularly suited for covalently bonded materials that require high temperatures to be produced with good crystallinity. Several years ago, we showed that plasma produced silicon nanocrystals are capable of high-efficiency photoluminescence, different from bulk silicon material. More recently, the capability of nonthermal plasmas to produce substitutionally doped nanocrystal materials has attracted attention, as substitutional doping had presented a significant challenge both for liquid and gas phase synthesis due to effects such as self-purification.

This presentation discusses the physics of plasma synthesis process. High photoluminescense quantum yields are achieved by careful surface functionalization through grafting alkene ligands to the nanocrystal surfaces. We also discuss the substitutional doping of silicon nanocrystals with boron and phosphorous using a nonthermal plasma technique. While the synthesis approach is identical in both cases, the activation behavior of these two dopants is found to be dramatically different. Finally, we present some experimental work on transport in films of highly phosphorous-doped nanocrystals, which indicates the approach to the metal-to-insulator transition.

This work was supported in part by the NSF Materials Research Science and Engineering Center under grant DMR-1420013, the DOE Energy Frontier Research Center for Advanced Solar Photophysics, and the Army Office of Research under MURI grant W911NF-12-1-0407.

Subject: Interaction of electrons with collective excitations in conventional and unconventional superconductors

Please note the date and time change this week.

Interaction of electrons with collective modes plays a central role in theory of conventional superconductors and many models of unconventional superconductors. In cuprates for example, presence of such interaction was discovered almost two decades ago [1, 2], however there is no consensus as to its origin [2-5]. It is not even known whether it is responsible for driving the pairing or a mere spectator of the superconducting transition. One of the reasons for this situation is lack of momentum resolved data from classical superconductors that would serve as a baseline for identifying the physical nature of the coupling interaction and its role in the pairing mechanism. In this talk we will present such data from classical BCS superconductor MgB2 [6, 7] and review the old and new properties of the collective mode present in cuprates. In the classical superconductor, the coupling of the electrons to the phonon mode responsible for pairing remains almost unaffected across the critical temperature. This is unlike the situation in cuprates, where the signature of coupling to the collective mode vanishes above Tc. We also discovered new low energy spectral feature in MgB2 that is most likely a signature of coupling to the Leggett mode – a relative phase excitation of the two suprefluids present in this material.

The cycling of gas through galactic fountains links disks to halos. Here I use a suite of high-resolution simulations of galaxy formation to follow the outflow and re-accretion of gas. These simulations self-consistently generate outflows from the available supernova energy in a manner that successfully reproduces key galaxy observables including the stellar mass-halo mass, Tully-Fisher, and mass-metallicity relations. From them one can quantify the importance of ejective feedback, relative to the efficiency of gas accretion and star formation, to setting the stellar mass. I show that ejective feedback is increasingly important as galaxy mass decreases and that the effective mass loading factor is independent of redshift. By examining spatially-resolved gas loss and accretion, I show the extent to which outflows redistribute baryons through the disk and what fraction of outflowing material is later recycled.

Subject: Variational Principle for Probabilistic Measure in Theory of Materials with Random Microstructures and Beyond

Materials with random microstructures can be adequately described only in probabilistic terms. One of the problems that arise is: how to find probabilistic characteristics of local fields (stresses, currents, etc.), if probability distributions of material characteristics are known? This is one of the two topics of the talk. The second topic is closely related to the first one and concerned thermodynamics of microstructure evolution. It will be argued that classical thermodynamics is not sufficient for macroscopic description of solids, and there is one more law of thermodynamics which controls evolution of microstructures.

In 1850, John Tyndall was struggling to find his way: he was a thirty-year old graduate student in mathematics living in an attic in Marburg, completely unknown, utterly broke, and working himself to the brink of mental and physical exhaustion. Within three years, he held a distinguished professorship at the Royal Institution of Great Britain, working alongside the likes of Michael Faraday, and commanding sold-out crowds wherever he lectured. I will follow Tyndall for these three years as he rose from obscurity to stardom. The route he took will sound both surprising and familiar to modern ears. How he navigated his way through the social and scientific world of mid Victorian Britain can tell us much about how a physicist is created, both then and now.

Subject: Higgs bosons and superconductors in the lab and throughout the universe

Refreshments to be served outside 230 SSTS after the colloquium.

I take an historical look at the developments of elementary
particle physics and condensed matter physics over the last century,
with focus on the interaction of these two grand areas of physics in
the context of spontaneous symmetry breaking. In particular, I compare
the discoveries of mutual interest in superconductivity and the
electroweak theory of particle physics and how the fields have
inspired each other. I describe how important the discovery of the
Higgs boson has been to particle physics and what it means for the
future. In the process I give an in-depth response to Anderson’s
recent statement in Nature: “Maybe the Higgs boson [of particle
physics] is fictitious!”

It is generally assumed that neutrino wave packets (WPs) do not overlap when they emerge from the source. We examine this assumption by modeling neutrinos as Gaussian WPs. A 3D solution with proper spherical shape has been derived to describe the propagation of a massless Gaussian WP. By using this 3D solution, we define the volume "occupied" by each WP to be the region in which the probability of finding the neutrino is 90%. A numerical factor is further defined as an indicator of the extent to which the WPs overlap in space. In particular, we consider the overlap among those neutrinos with mean energies differing by no more than the intrinsic energy uncertainty in a WP. The overlap factor depends on how sharp the transverse momentum distribution of the initial WP is and the differential production rate of the neutrino source. The estimate of the factor suggests that such WP overlap is significant for solar and supernova neutrinos and is potentially significant for neutrinos from radioactive sources and fission reactors. We raise the question of whether the ferminonic nature of neutrinos affect experimental observables if such overlap is significant.

Semimetals and Spin Liquids are two sought-after states in modern condensed matter physics. In this talk I will discuss the Kitaev spin liquid and its excitations. I will then present Raman scattering as a probe for these exotic states in candidate spin-orbit coupled materials. It turns out that Raman scattering probes a fermionic quasiparticle in this system that realizes effective semimetal physics. Finally, I will speculate about Raman signatures of semimetal surface modes.

After the Higgs boson discovery we are able to refocus our efforts on precision analysis in search of new physics contributions now that the Standard Model and its parameters are known well. Some questions I will address concern self-consistent precision analysis frameworks, new physics expectations for deviations, and comparisons with experimental sensitivities for currently running and future facilities.

In 1618, an alchemical savant named Michael Maier published an extraordinary alchemical emblem book, the Atalanta fugiens. Distinguished among the hermetic corpus for its fifty exquisite engravings of enigmatic alchemical images, which are set to music, Maier's Atalanta fugiens is an elegant audio-visual articulation of alchemical theory and practice for producing the philosophers’ stone, the panacea held to restore perfect health and longevity to humankind. The Atalanta fugiens is Maier's allegorical paean to wisdom achieved through the pursuit of true alchemy, and it is his evocation of a Golden Age published on the eve of the Thirty Years' War. However, this book contains a secret that has lain hidden for the past four hundred years, enciphered in its pages. This talk explores Maier's Atalanta fugiens as a virtuoso work of allegorical encryption that fuses poetry, iconography, music, mathematics, and Christian cabala to extol hermetic wisdom, while evoking alchemical technologies and laboratory processes - for Maier’s emblem book functions as a game or puzzle that the erudite reader must solve, decode, play.

Radio relics are believed to trace Mpc-scale shocks in the outskirts of massive galaxy clusters. They are likely sites of particle acceleration, where cosmic-ray electrons gain energy in the shock and become synchrotron bright in the magnetic field of the intra-cluster medium. Many models for radio relics require substantially larger magnetic fields than motivated from our general understanding of fields and their amplification in the outskirts of cluster. We present a new detailed model of the prototype of radio relics, the Sausage relic in CIZA J2242.8+5301, aimed to reproduce the recently observed spectral steepening in the relic. We also provide limits on the acceleration efficiency
required to model the relic, which puts tension on direct shock acceleration from the thermal pool.

When a superconductor (S) and a ferromagnet (F) are put into contact with each other, the combined S/F hybrid system exhibits altogether new properties. There is a proximity effect where electron pair correlations from S penetrate into F, but the pair correlations oscillate rapidly and decay over a very short distance due to the large exchange splitting between the spin-up and spin-down electron bands in F. In the presence of non-collinear magnetization, Bergeret et al. predicted that spin-triplet pair correlations are generated, which are immune to the exchange field and hence persist over much longer distances in F [1]. Furthermore, these triplet pair correlations satisfy the Pauli Exclusion Principle in a new and strange way: they are odd in frequency or time. Several groups have now observed convincing evidence for such spin-triplet correlations in a variety of S/F and S/F/S systems. Our approach is based on measuring the supercurrent in Josephson junctions of the form S/F’/F/F’’/S, with non-collinear magnetizations in adjacent ferromagnetic layers [2]. I will discuss our latest results toward controlling the supercurrent in these junctions [3], as well as how various types of ferromagnetic junctions could be used as memory elements in a fully superconducting random-access memory [4].

Work supported by the US DOE under grant DE-FG02-06ER46341, by Northrop Grumman Corporation, and by IARPA via U.S. Army Research Office contract W911NF-14-C-0115.

The observed neutrino flavor transitions are currently explained by the three flavor neutrino oscillation phenomenon, considered to be the leading order mechanism behind the flavor transitions. Currently existing data from LSND, miniBooNE and reactor experiments demonstrate anomalies that could potentially be indications of non-standard neutrino phenomena. MINOS can probe transitions from muon neutrinos to electron neutrinos and search for anomalous behavior that cannot be explained by standard model neutrino oscillations. I will present the search for second order effects in the flavor transitions by analyzing the MINOS
νμ→νe
channel.

Have you discovered a planetary system today? If not, don't worry. At DiskDetective.org, you can help scour the WISE data archive to find new planetary systems and candidate advanced extraterrestrial civilizations. Volunteers at this new citizen science website have already performed almost 1.5 million classifications of WISE sources, searching a catalog 8x the size of any previously published survey. Through a follow-up spectroscopic and imaging campaign in both hemispheres we are re-observing many user-classified Objects of Interest to further vet them for background contaminants and derive better spectral types. Come by to hear about our growing catalog of candidate debris disks, protoplanetary disks, and other interesting objects with 22 micron excess all around the sky--and some strange stories about doing science with 28,000 helpers.

The story of Fermi's "little neutral ones" has already many surprises and inspiring examples of daring experimental initiative. Today a host of new experiments are trying to unlock the secrets of these elusive particles.

Using fluid/gravity correspondence, we study all-order resummed hydrodynamics in a weakly curved spacetime. The underlying microscopic theory is a finite temperature \mathcal{N}=4 super-Yang-Mills theory at strong coupling. To linear order in the amplitude of hydrodynamic variables and metric perturbations, the fluid's stress-energy tensor is computed with derivatives of both the fluid velocity and background metric resummed to all orders. In addition to two viscosity functions, we find four curvature induced structures coupled to the fluid via new transport coefficient functions, which were referred to as gravitational susceptibilities of the fluid (GSF). We analytically compute these coefficients in the hydrodynamic limit, and then numerically up to large values of momenta. We extensively discuss the meaning of all order hydrodynamics by expressing it in terms of the memory function formalism, which is also suitable for practical simulations. We also consider Gauss-Bonnet correction in the dual gravity, which is equivalent to some 1/N corrections in the dual CFT. To leading order in the Gauss-Bonnet coupling, we find that the memory function is still vanishing.

In order to explain the baryon asymmetry of the Universe, we need extra CP violation, and an out-of-equilibrium process. Supersymmetric theories with a global U(1) symmetry solve the SUSY flavor and CP problems. They also have pseudo-Dirac gauginos with particle--antiparticle oscillations. If the gauginos decay out-of-equilibrium, and if there is CP violation in the oscillations, can they produce the baryon asymmetry? You should come and learn!

Global climate is undergoing change now. In the geologically recent past, global climate has experienced large swings as ice sheets waxed and waned, causing global sea level to fluctuate by ~120 m. These changes are accompanied by significant changes in the CO2 content of the atmosphere, indicating a close but incompletely understood link between CO2 and global climate. It is also unclear why and how atmospheric CO2 can vary naturally by as much as it did in the recent past. This presentation will explore these issues and attempt to put the ongoing climate change in perspective.

At least until Hubbell’s neutral theory emerged, no concept was thought more important to theorizing in ecology than the niche. Without it––and its highly abstract definition by Hutchinson in particular––technically sophisticated and well-regarded theories of character displacement, limiting similarity, and many others would seemingly never have been developed. The niche concept is also the centerpiece of perhaps the best candidate for a distinctively ecological law, the competitive exclusion principle. But the incongruous array of proposed definitions of the concept squares poorly with its apparent centrality. I argue this definitional diversity reflects a problematic conceptual imprecision that challenges its putative indispensability in ecological theory. Recent attempts to integrate these disparate definitions into a unified characterization fail to resolve the imprecision.

Surveys of the CMB from ground-based observatories have revealed much about cosmology, in particular by measuring effects on CMB photons since recombination. In this talk, I will summarize results from the ongoing ACTPol survey, highlighting in particular the measurement of gravitational lensing by matter between us and the CMB recombination surface. I will discuss challenges with these measurements and also look forward to what will be possible with upcoming surveys such as SPT-3G, AdvACT, and CMB-S4.

This is the public portion of Mr. Albright's thesis defense. His advisor is Joseph Kapusta

The universe is filled with protons and neutrons, which are themselves made of quarks and gluons. However, microseconds after the Big Bang, the universe was so hot and dense that quarks and gluons existed in other phases--first as a quark-gluon plasma, then as a hadron gas. Remarkably, these exotic phases of matter are created and studied experimentally in heavy-ion collisions at particle accelerators like RHIC at Brookhaven National Lab and the LHC at CERN. There is a strong effort underway to develop precision models of heavy-ion collisions which, combined with experimental data, will enable improved determination of many physical properties of quark-gluon matter. Recent years have seen significant improvements in modeling matter with zero net baryon density, which is relevant for RHIC, the LHC, and the early universe. However, there is growing interest in matter with a large net baryon density, which is relevant for neutron stars and future collider experiments. Hence, in this thesis we compute various thermodynamic properties of matter at large baryon densities. We first improve the popular hadron resonance gas equation of state (EoS) by including repulsive interactions, which are important at large baryon densities. Next, we develop crossover equations of state which smoothly transition from hadron gas models at low energy densities to a quark-gluon plasma EoS at high energy densities. We then compute net baryon number fluctuations and compare them to measurements from the STAR Collaboration at RHIC. We find that fluctuations freeze out long after chemical reactions do. Finally we develop a relativistic quasiparticle kinetic theory of hadron gases at large baryon densities. This includes interactions via scalar and vector mean fields in a thermodynamically consistent way. We then derive formulas for the shear and bulk viscosity and thermal conductivity, which are interesting quantities that influence experimental observables.

Subject: Density slope oscillations in the central regions of galaxy and cluster-sized systems

The distribution of matter in halos has a variety of consequences ranging from the ability to verify dark matter models through structure growth to holding clues about the physical processes shaping and forming halos and the galaxies within. We present data from two papers describing the central regions of early type galaxies and galaxy clusters and along with dark matter N-body simulations, analyze and comment on "oscillations" present in central regions of their radial, total density profiles.

We calculate the anomalous Hall conductivity of surface states on three dimensional topological Kondo insulators with cubic symmetry and multiple Dirac cones. We treat a generic model in which the Fermi velocity, the Fermi momentum and the Zeeman energy in different pockets may be unequal and in which the microscopic impurity potential is short ranged on the scale of the smallest Fermi wavelength. Our calculation of AHE to the zeroth (i.e. leading) order in impurity concentration is based on the Kubo-Smrcka-Streda diagrammatic approach. It also includes certain extrinsic skew scattering contributions with a single cross of impurity lines, which are of the same order in impurity concentration. We discuss various special cases of our result and the experimental relevance of our study in the context of recent hysteretic magnetotransport data in SmB6 samples.

Current and future neutrino oscillation experiments will depend on accurate neutrino interaction models in order to measure oscillation parameters and explore for CP violation. To support this, recently updated models for neutrino-nucleus interactions are compared to new cross sections measured by the MINERvA experiment. These results cover low momentum transfer events from quasi-elastic to resonance production, which are the most important for oscillation measurements. Discussion will include the impact on oscillation experiments and also the neutrino view of these multi-nucleon effects.

Researchers estimate that more PhD physicists in the US work in the private sector than in academia, so thinking through an industrial career path is a very useful exercise for current physics graduate students. In this talk, Dr. Wilkens will present recent career statistics of physicists in the private sector, along with the story of her industrial career after receiving her PhD from the U of MN Physics Department in 2003. She will also share thoughts on the industrial career paths chosen by her colleagues, and delve into the new and exciting careers she sees opening up for physicists in less traditional sectors.

I will briefly review the Color Glass Condensate framework, which was proposed to describe the high energy limit of hadronic wave functions. Its classical realization, the McLerran-Venugopalan[MV] model will be used to study the early time dynamics of relativistic heavy ion collisions. Within the MV model, we solve the classical Yang-Mills equations analytically via a near-field(small-\tau) power series expansion. Energy-momentum tensor of the gluon field is calculated to all orders under a leading Q^2 approximation. Our calculations favor an early pressure isotropization at the time of the scale Q_s^{-1}.

Freeze-in is a general and calculable mechanism for dark matter production in the early universe. Assuming a standard cosmological history, such a framework predicts metastable particles with a lifetime generically too long to observe their decays at colliders. In this talk, I will consider alternative cosmologies with an early matter dominated epoch, and I will show how the observed abundance of dark matter is reproduced only for shorter lifetimes of the metastable particles. Famous realization for such a cosmology are moduli decays in SUSY theories and inflationary reheating. Remarkably, for a large region of the parameter space the decay lengths are in the displaced vertex range and they can be observable at present and future colliders. I will conclude with an example of DFSZ SUSY theories where this framework is realized.

The "supernova impostors" resemble the appearance of a true supernova, but
rather than a terminal explosion of a star, the impostors appear to be massive stars that have undergone a giant eruption and survived. Several of these have energetics comparable to true supernovae, and may be analogous to the Great Eruption of the massive star Eta Carinae in the 1800s.
I discuss observed characteristics of SN impostors and what is presently known and unknown about them. I present new numerical simulations following the recovery of the star from a giant eruption. The numerical results show that the eruption is a runaway event that causes huge mass loss from the star. The simulated star develops inner pulsations that further drive stellar wind with strong mass loss, as observed in erupting objects of that kind. It takes the star a few centuries to recover from the eruption and return to equilibrium.

In the preface to his 1655 De Corpore, Thomas Hobbes identified William Harvey as the first to discover and demonstrate the science of the human body, and set him alongside Copernicus and Galileo as a founder of genuine natural science. Hobbes says Harvey is the only man he knows who, conquering envy, established a new doctrine in his own lifetime. Harvey himself frames his De motu cordis (1628) as an effort, both methodologically sound and morally upright, to convince “studious, good, and honest men” despite the ill will and machinations of those with biased minds. Drawing on his anatomy lecture notes, I first unpack Harvey’s understanding of right method in “philosophical anatomy.” I then trace how this understanding shapes Harvey’s argumentation in the De motu cordis, including its moral valence.

Quantum phases of matter that violate time-reversal symmetry invariably develop local spin or orbital moments in the ground state, such as ferromagnets, spin density waves, electrons in loop current phases, etc. A common property is that time reversal symmetry is restored as soon as the moments melt. However, there may exist a phase of matter that violates time-reversal symmetry but has no moments, which is called the “directional scalar spin chiral order” (DSSCO) from Ref. [1]. It can be obtained by melting the spin moments in a magnetically ordered phase but retaining residual broken time-reversal symmetry.

Subject: How many electrons make a semiconductor nanocrystal film metallic

For films of semiconductor nanocrystals to achieve their potential as novel,
low-cost electronic materials, a better understanding of their doping to tune
their conductivity is required. So far, it not known how many dopants will
turn a nanocrystal film from semiconducting to metallic. In bulk
semiconductors, the critical concentration of electrons at the metal-
insulator transition is described by the famous Mott criterion. We show
theoretically that in a dense NC film, where NCs touch each other by small
facets, the concentration of electrons N at the metal-insulator transition
satisfies the condition: N r^3 = 0.3, where r is a radius of contact facets.
In the accompanying experiments, we investigate the conduction mechanism in
films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest
electron concentration achieved in our samples, which is half the predicted
N, we find that the localization length of hopping electrons is close to three
times the nanocrystals diameter, indicating that the film approaches the
metal-insulator transition.

This is the public portion of Mr. Li's thesis defense. His adviser is Mikhail Voloshin.

Recent experiments have brought us new surprises about old quarkonium system. A series of exotic resonances are discovered in electron-positron colliders and new hadronic processes are measured. I will discuss some aspects of those new experimental results. The heavy quark spin symmetry and QCD-based method will be used to understand those results.

A stability analysis of out of equilibrium and boundary driven systems is presented. It is performed in the framework of the hydrodynamic macroscopic fluctuation theory and assuming the additivity principle whose interpretation is discussed with the help of a Hamiltonian description. An extension of Le Chatelier principle for out of equilibrium situations is presented which allows to formulate the conditions of validity of the additivity principle. Examples of application of these results in the realm of classical and quantum systems are provided.

A Type-I X-ray burst is the thermonuclear runaway that occurs on the surface of a neutron star in a binary system. Studies on these bursts are of great importance for understanding neutron stars, nuclear reactions and the equation of state of dense matter at low temperature. I will discuss a subset of X-ray bursts, photospheric radius expansion bursts, that is powerful to lift up the photosphere of the star with the simulations based on a new 1D turbulence model, ODT. The model is different in that the turbulent motion is implemented according to a stochastic process and an eddy event is represented by a measure-preserving map. I will compare the light curve, abundances, and turbulent motion development with a KEPLER model in which the traditional mixing length theory is applied. The light curves of both models will be compared with observational data.

If dark matter is embedded in a non-trivial hidden sector, it may annihilate and decay to lighter hidden sector states which subsequently decay to Standard Model particles. While remaining agnostic to the details of the hidden sector model, our framework - with annihilations followed by cascading hidden sector decays - captures the generic broadening of the spectrum of secondary particles (photons, neutrinos, electron-positrons, and antiprotons) relative to the case of dark matter annihilating directly to Standard Model particles. I will detail how such scenarios can explain the apparent excess in GeV gamma-rays identified in the central Milky Way, while evading bounds from direct detection experiments. Additionally I will describe how indirect constraints on dark matter annihilation limit the parameter space for such cascade/multi-particle decays. In particular I will describe an investigation of the limits from the cosmic microwave background by Plank, the Fermi measurements of photons from the Milky Way Dwarf Spheroidal Galaxies, and positron data from AMS-02. Generally the bound from the Fermi dwarfs is the most constraining for annihilations to photon-rich final states, while AMS-02 is most constraining for electron and muon final states; however in certain instances the CMB bounds overtake both, due to their approximate independence of the details of the hidden sector cascade.

Galactic structure is a topic that is so old that it’s become new again. After a brief review of history and recent events in Galactic structure. I will discuss two (and a half) new results about the overall structure of the Galaxy that come principally from two on-going Galactic surveys at Wisconsin: GLIMPSE and WHAM. GLIMPSE (Galactic Legacy Infrared Midplane Survey Extraordinaire) is a high angular resolution (arcsec) mid-infrared survey of the Galactic midplane using the Spitzer Space Telescope. From this survey, I will present some new results from this survey on the non-axisymmetric structure of the stellar disk. WHAM is a lower angular resolution (degree) velocity-resolved H-alpha survey of the diffuse ionized gas in the Galaxy (and beyond). From this survey, I will present some new information about our local solar neighborhood and a potential large scale magnetized outflow. (The reason for the non-integer number of discoveries is that one of the things we’ve found about the Milky Way is known to some, but not generally accepted by all.)

Speaker: Nicholas Buchanan, History of Science and Technology, University of Minnesota

Subject: Tanked: On Keeping This Alive in Places They Shouldn't Be

Refreshments served at 3:15 p.m.

In this talk, Dr. Buchanan will discuss the history of two tanks, each of which was designed to keep organisms alive in places where they otherwise would have perished. These artificial environments—aquaria beginning in the mid-19th century and spacecraft in the mid 20th—together offer a window onto changing perceptions about the human ability to know the natural world and to use that knowledge to control, manipulate, and even replicate it. In both cases, scientists, engineers, and enthusiasts used changing knowledge about the earth and its inhabitants to create technologies that were meant to be “an imitation of the means employed by nature herself” (to use the words of a Victorian aquarian), ranging from table-top jars to large institutional aquaria, from single-person capsules to plans for permanent human colonies in space. I’ll argue that building artificial environments was an important activity from which scientists, engineers, the public, and policy-makers learned about the systemic complexities of nature. What’s more, the difficult task of making artificial environments that could actually support life for long periods, and the ease with which these could be “broken,” highlighted the fragility of nature and its vulnerability to human intervention.

The origin of cosmic high-energy neutrinos is a new mystery in astroparticle physics. We discuss possible scenarios and general implications that have been obtained so far, showing the importance of multi-messenger data. Then, we summarize open questions and future prospects.

Subject: The Emergence of Superconductivity in Inhomogeneous, Mesoscopic Systems

Although low-dimensional, inhomogeneous superconductors have been intensely studied, the nature of the onset of superconductivity in these systems is still largely unknown. In this talk I will present transport measurements on mesoscopic disks of granular, inhomogeneous Nb, where the superconducting transition temperature is determined as a function of disk diameter. We observe an unexpected suppression of superconductivity at micron diameters, length scales that are considerably longer than the coherence length of Nb. This suppression does not appear in large-scale films, and cannot be explained by single-grain small-size effects. By considering the diameter-dependence of the transition, as well as observations of strong fluctuations in the transition temperature as disk diameters decrease, we are able to explain this long length scale dependence by an extremal-grain model, where superconducting order first appears in unusually large grains and, due to proximity coupling, spreads to other grains. The extremal-grain onset of superconductivity has not previously been observed experimentally, and explains how superconductivity can emerge in granular or inhomogeneous superconductors.